Question 1

Unit equivalence for dynamic viscosity: 1 centipoise (cP) is approximately equal to which of the following?

Accepted Answer

Introduction / Context: Dynamic viscosity appears in momentum transfer, pump sizing, and mixing problems. Engineers often need to convert between SI and English units. Knowing reliable equivalences for 1 centipoise (cP) prevents conversion errors. Given Data / Assumptions: 1 poise (P) = 1 g/(cm·s). 1 centipoise (cP) = 0.01 P = 0.01 g/(cm·s). In SI, 1 cP = 1 mPa·s = 0.001 Pa·s. Concept / Approach: We convert 1 cP (0.001 Pa·s) to lb/(ft·hr) to match the offered options. Use the chain: Pa·s → kg/(m·s) → lb/(ft·hr). This conversion yields a convenient benchmark used widely in handbooks. Step-by-Step Solution: 1 cP = 0.001 Pa·s = 0.001 kg/(m·s).Convert to lb/(ft·hr): multiply by (2.20462 lb/kg) / (3.28084 ft/m) × 3600 s/hr.Result ≈ 2.42 lb/(ft·hr). Verification / Alternative check: Also remember equivalences: 1 cP = 0.01 g/(cm·s) and 1 cP = 1 mPa·s. While both are correct statements, only one of the given numerical English-unit options matches the standard value, namely 2.42 lb/(ft·hr). Why Other Options Are Wrong: 1 g/(cm·s): This is 1 P, not 1 cP. 1 cSt: Kinematic viscosity; equals 1 cP only when density = 1 g/cm³ (not a general equivalence). 2.42 lb/(ft·s): Off by a factor of 3600 (seconds vs hours). 1 mPa·s: Although true, it was not among the original options in many legacy sets; if present, it would also be correct. Here the targeted numeric English-unit match is 2.42 lb/(ft·hr). Common Pitfalls: Confusing dynamic with kinematic viscosity; dropping the hour-to-second conversion; mistaking poise for centipoise. Final Answer: 2.42 lb/(ft·hr)

Question 2

Concentration unit definition: The number of gram-moles of solute dissolved per litre of solution is called…

Accepted Answer

Introduction / Context: Different analytical and process problems call for different concentration measures. Molarity is ubiquitous in laboratory work and reaction-rate expressions because it references the actual solution volume at the condition of interest. Given Data / Assumptions: Molarity (M) = moles of solute per litre of solution. Other units exist (molality, normality, formality) and should not be confused with molarity. Temperature can change solution volume, hence molarity can be temperature-dependent. Concept / Approach: Identify the unit that matches “gram-moles per litre of solution.” This maps directly to molarity. Molality uses kilograms of solvent; normality uses equivalents per litre; equivalent weight is not a concentration unit. Step-by-Step Clarification: Molarity M = n_solute / V_solution(L).Molality m = n_solute / mass_solvent(kg).Normality N = equivalents / litre (context-dependent on reaction). Verification / Alternative check: Check typical notations: 1.0 M HCl means 1 mole HCl in each litre of final solution volume, consistent with the definition. Why Other Options Are Wrong: Equivalent weight: Property of a substance, not a concentration unit. Molality/Normality/Formality: Do not match the “per litre of solution” wording. Common Pitfalls: Interchanging molarity and molality; ignoring temperature effects on solution volume when high precision is needed. Final Answer: molarity

Question 3

Total energy at a point in a flowing system: Which energies are included when considering the total specific energy of a fluid stream at a point?

Accepted Answer

Introduction / Context: In the steady-flow energy equation and Bernoulli-like formulations extended to real fluids, the total specific energy of a stream at a point is the sum of several contributions. Recognising all components prevents omissions in pump, turbine, and heat-exchanger calculations. Given Data / Assumptions: Single-phase fluid stream at a defined point in a control volume. Negligible chemical reaction or phase change at the point considered. Classical thermodynamic definition of properties applies. Concept / Approach: Total specific energy e_total typically comprises internal energy u, kinetic energy v^2/2, potential energy gz, and flow (pressure) energy p/ρ. In some texts, enthalpy h = u + p/ρ is used, so e_total = h + v^2/2 + gz. Regardless of representation, all four physical contributions are present. Step-by-Step Clarification: Internal energy (microscopic modes) = u.Flow/pressure energy = p/ρ (work required to push fluid into the control volume).Kinetic energy = v^2/2 (per unit mass).Potential energy = g*z (per unit mass). Verification / Alternative check: Steady-flow energy equation per unit mass: h1 + v1^2/2 + g z1 + q_in − w_out = h2 + v2^2/2 + g z2. Here h = u + p/ρ incorporates internal and pressure energies, confirming their inclusion. Why Other Options Are Wrong: Options listing only one or two forms omit necessary contributors. Pressure-only or internal-only simplifications are incomplete except in very restricted comparisons that hold other terms constant. Common Pitfalls: Forgetting to include elevation head in tall columns; neglecting kinetic terms at high velocities; double-counting p/ρ when using enthalpy versus internal energy. Final Answer: all: potential, kinetic, pressure (flow) and internal

Question 4

Unit conversion in thermodynamics: 1 BTU per cubic foot is approximately equal to how many kilocalories per cubic meter?

Accepted Answer

Introduction / Context: Unit conversion is a foundational thermodynamics skill for chemical and mechanical engineers. This question asks you to convert an energy density expressed in British units (BTU per cubic foot) to SI-compatible units (kilocalories per cubic meter). Such conversions are common when comparing fuel values, heating values, or designing energy systems where consistent units are essential. Given Data / Assumptions: 1 BTU ≈ 252 cal = 0.252 kcal. 1 ft^3 = 0.0283168 m^3. We assume standard definitions without temperature-dependent corrections. Concept / Approach: Convert the numerator (energy) and the denominator (volume) separately and then combine. The goal is to express BTU/ft^3 as kcal/m^3 by multiplying by appropriate conversion factors that equal 1 in value but change units. Step-by-Step Solution: Start with 1 BTU/ft^3.Convert energy: 1 BTU = 0.252 kcal.Convert volume: 1 ft^3 = 0.0283168 m^3.Compute: (0.252 kcal) / (0.0283168 m^3) = 8.9 kcal/m^3 (approximately).Round sensibly to 1 significant figure consistent with options: ≈ 9 kcal/m^3. Verification / Alternative check: Since 1 m^3 is ~35.3147 ft^3, 1 BTU/ft^3 equals 35.3147 BTU/m^3. Multiply by 0.252 kcal/BTU: 35.3147 * 0.252 ≈ 8.9 kcal/m^3. This matches the earlier computation. Why Other Options Are Wrong: 1 kcal/m^3: Underestimates by roughly a factor of 9. 4 kcal/m^3: About half the correct value. 252 kcal/m^3: Confuses BTU-to-cal conversion without accounting for volume. 0.252 kcal/m^3: Converts BTU to kcal but ignores ft^3 to m^3 scaling. Common Pitfalls: Mixing up 1 BTU = 252 cal with 252 kcal, forgetting to convert ft^3 to m^3, or inverting the volume ratio. Always keep track of units explicitly in every step. Final Answer: 9 kcal/m^3

Question 5

Vapor–liquid equilibrium (Raoult’s law): Calculate the total pressure of a liquid mixture (benzene, toluene, o-xylene) at 100 °C given their pure-component vapor pressures.

Accepted Answer

Introduction / Context: This problem tests application of Raoult’s law for an ideal liquid solution at a fixed temperature. The total pressure above the mixture equals the sum of each component’s partial pressure, which is the product of its liquid mole fraction and its pure-component vapor pressure at that temperature. Given Data / Assumptions: Liquid moles: benzene = 0.45 kmol, toluene = 0.44 kmol, o-xylene = 0.23 kmol. Total moles in liquid = 0.45 + 0.44 + 0.23 = 1.12 kmol. P^sat at 100 °C (mmHg): benzene = 1340, toluene = 560, o-xylene = 210. Assume ideal solution behavior (Raoult’s law) and negligible gas-phase non-ideality. Concept / Approach: For each component i: p_i = x_i * P_i^sat, where x_i is the liquid mole fraction. The total pressure P_total = Σ p_i across all components. Step-by-Step Solution: Compute x_benzene = 0.45 / 1.12 = 0.401786.Compute x_toluene = 0.44 / 1.12 = 0.392857.Compute x_oxylene = 0.23 / 1.12 = 0.205357.Partial pressures: p_B = 0.401786 * 1340 ≈ 538.39 mmHg.p_T = 0.392857 * 560 ≈ 220.00 mmHg.p_X = 0.205357 * 210 ≈ 43.13 mmHg.Total pressure P_total = 538.39 + 220.00 + 43.13 ≈ 801.52 mmHg. Verification / Alternative check: Recalculate with rounded x_i values to two or three decimals; the total remains ≈ 801.5 mmHg, confirming robustness to small rounding differences. Why Other Options Are Wrong: 756.2 / 780.5 mmHg: Underestimates; likely from arithmetic or mole-fraction rounding errors. 880.5 / 900.0 mmHg: Overestimates; would require higher P^sat weighting or incorrect x_i. Common Pitfalls: Forgetting to normalize mole fractions, mixing activity coefficients into an ideal law problem, or confusing mass fractions with mole fractions. Final Answer: 801.5 mmHg

Question 6

Vapor states and definitions: Identify the incorrect statement among standard definitions of saturated and superheated vapor.

Accepted Answer

Introduction / Context: Understanding precise thermodynamic terminology is critical in phase-equilibrium calculations and equipment design. This item checks your familiarity with saturated vapor, superheated vapor, boiling point, and dew point, and asks you to spot a subtle but important misuse of terms. Given Data / Assumptions: Standard definitions of saturation, superheat, boiling point, and dew point are used. Pure-substance behavior is implied unless otherwise specified. Concept / Approach: A saturated vapor exists at its equilibrium vapor pressure for the given temperature (or equivalently at its saturation temperature for the given pressure). Superheat is the temperature above saturation at the same pressure. Dew point is the temperature at which a gas mixture just begins to condense at a given pressure; it is a mixture concept, not the definition of pure-substance saturation temperature. Step-by-Step Solution: Check (a): Correct by definition.Check (b): Correct; if partial pressure is less than equilibrium vapor pressure, the vapor is superheated (undersaturated).Check (c): Correct for a pure substance: saturation temperature at a specified pressure is the boiling point.Check (d): Incorrect wording: degree of superheat is referenced to the saturation temperature at the same pressure. Calling that saturation temperature the “dew point” is a mixture-specific term and not correct for a single-component vapor. Verification / Alternative check: For pure water at 1 atm, saturation temperature is 100 °C. If steam at 1 atm is at 120 °C, degree of superheat is 20 °C. The “dew point” is used for air–water vapor mixtures; using it for pure steam is not standard. Why Other Options Are Wrong: (a), (b), and (c) are standard, accepted definitions. (e) is invalid because (d) is not fully correct. Common Pitfalls: Confusing dew point (mixture property) with saturation temperature of a pure component; mixing pressure- and temperature-specified states; and forgetting the “same pressure” condition when defining superheat. Final Answer: The degree of superheat equals the difference between the vapor’s temperature and its saturation temperature (i.e., dew point).

Question 7

Thermochemistry concept check: Hess’s law of constant heat summation deals with what fundamental quantity or relationship?

Accepted Answer

Introduction / Context: Hess’s law is a cornerstone of thermochemistry. It allows chemists and engineers to compute heats of reaction that may be difficult to measure directly by algebraically combining reactions whose enthalpies are known, because total enthalpy change depends only on the initial and final states—not on the path taken. Given Data / Assumptions: Chemical reactions are considered at a common reference state. Enthalpy is a state function; path independence applies. Concept / Approach: Hess’s law states that the total heat of reaction for a process is the sum of the heats of reaction for any set of steps into which the overall reaction can be divided. This law directly concerns changes in heat of reaction (enthalpy change, ΔH), not kinetics or equilibrium constants. Step-by-Step Solution: Identify what Hess’s law governs: energy (enthalpy) changes of reactions.Eliminate kinetics: reaction rate is not addressed by Hess’s law.Eliminate equilibrium: equilibrium constants relate to Gibbs free energy and temperature, not directly to Hess’s construction.Conclude that “changes in the heat of reaction” is the correct choice. Verification / Alternative check: Example: If A → B has ΔH1 and B → C has ΔH2, then A → C has ΔH = ΔH1 + ΔH2, independent of the path. This illustrates Hess’s law in action. Why Other Options Are Wrong: Equilibrium constant: related to ΔG° and temperature via van ’t Hoff; not Hess’s law. Reaction rate: kinetics are unrelated to enthalpy summation. Entropy change of formation: not the central subject of Hess’s law. None of these: incorrect because the law clearly handles heats of reaction. Common Pitfalls: Confusing path independence (thermodynamics) with rate dependence (kinetics), or mixing up ΔH with ΔG and K. Always distinguish thermodynamic state functions from kinetic behavior. Final Answer: Changes in the heat of reaction

Question 8

Process dynamics terminology: When is a chemical process said to be operating under unsteady (transient) state conditions?

Accepted Answer

Introduction / Context: In process control and mass/energy balance calculations, distinguishing steady state from unsteady (transient) state is fundamental. Most design correlations assume steady state, while start-up, shut-down, and disturbances are transient. Given Data / Assumptions: Generic process (reactor, separator, or heat exchanger) with inlets and outlets. System boundaries remain fixed. Concept / Approach: Steady state means that all macroscopic variables inside the control volume (e.g., holdup, composition, temperature) are time-invariant. Under unsteady state, at least one of these variables changes with time, often due to changing inflows/outflows or reactions not balanced by removal. Step-by-Step Solution: Identify time-dependence: if concentrations or flow rates change with time, the process is unsteady.Option (c) explicitly states that both flow and composition are time-dependent; this is a textbook indicator of unsteady state.Options (a) and (b) imply constant conditions; they correspond to steady-state behavior.Option (e) is incomplete; temperature alone may vary, but the definition emphasizes process variables broadly, including compositions and flows. Verification / Alternative check: Write a dynamic mass balance: d(Inventory)/dt = In − Out + Generation − Consumption. If d(Inventory)/dt ≠ 0 for any species or energy, conditions are unsteady. Why Other Options Are Wrong: (a) and (b): Describe time-invariant behavior. (d): Invalid because (c) correctly states unsteady state. (e): Too narrow; unsteady state typically involves broader time dependence. Common Pitfalls: Confusing periodic steady state with true unsteady state, or assuming thermal transients alone define dynamics. Always check material and energy inventories. Final Answer: When both flow rates and compositions vary with time

Question 9

Relative volatility intuition: Given equal normal boiling-point vapor pressures at 100 °C, which compound will have the higher vapor pressure at 150 °C—water or methylcyclohexane?

Accepted Answer

Introduction / Context: The Clausius–Clapeyron relation qualitatively explains how vapor pressure varies with temperature. Substances with lower latent heat of vaporization show a faster increase in vapor pressure with rising temperature. This question applies that insight to compare water with methylcyclohexane at 150 °C, given both have the same vapor pressure at 100 °C. Given Data / Assumptions: At 100 °C: P_sat,water = P_sat,methylcyclohexane = 1 atm. Latent heats (at ~100 °C): λ_water ≈ 40.63 kJ/mol; λ_methylcyclohexane ≈ 31.55 kJ/mol. At 150 °C: P_sat,water = 4.69 atm (given). Assume Clausius–Clapeyron holds approximately over this range and λ is weakly temperature dependent. Concept / Approach: Clausius–Clapeyron in integrated form suggests ln(P2/P1) ≈ −(λ/R) * (1/T2 − 1/T1). For a given temperature interval, smaller λ gives a larger increase in ln(P) and thus a higher P2. Step-by-Step Solution: Compare λ values: 31.55 kJ/mol (methylcyclohexane) < 40.63 kJ/mol (water).For the same Δ(1/T), ln(P2/P1) is inversely proportional to λ.Therefore, methylcyclohexane’s P_sat at 150 °C will exceed that of water at 150 °C.Given water’s P_sat(150 °C) = 4.69 atm, methylcyclohexane is expected to be > 4.69 atm. Verification / Alternative check: A rough calculation with constant λ would show a larger ln(P2/P1) for methylcyclohexane. While exact numbers require integration and real-property data, the qualitative conclusion is robust. Why Other Options Are Wrong: (a) and (b) contradict the λ-based trend. (d) We have sufficient comparative data (equal P at 100 °C and λ values) to draw a qualitative conclusion. (e) Exact equality at 150 °C is not expected because λ values differ. Common Pitfalls: Assuming equal vapor pressures at one temperature implies equal curves at all temperatures. Always consider latent heat differences. Final Answer: Significantly more than 4.69 atm (methylcyclohexane higher than water)

Question 10

Estimating heat capacity of solid compounds: Which empirical rule is commonly used to approximate the molar heat capacity of a solid from its constituent atoms?

Accepted Answer

Introduction / Context: Property estimation methods are vital when direct measurements are unavailable. For solids, one classical approach estimates the molar heat capacity from atomic contributions. Recognizing which rule applies helps in rapid preliminary calculations for materials and process design. Given Data / Assumptions: We seek an empirical rule for solid heat capacity estimation. Idealized additive behavior by atoms in a crystal lattice is assumed. Concept / Approach: Kopp’s rule approximates the molar heat capacity (at constant pressure) of a solid compound as the sum of the atomic heat capacities of its constituent elements, each multiplied by their stoichiometric coefficients. It is particularly useful at moderate temperatures where quantum effects are modest. Step-by-Step Solution: Identify target property: molar heat capacity of a solid compound.Recall named rules: Trouton’s rule relates to entropy of vaporization; Clapeyron to phase equilibria; Gibbs equation to thermodynamic potentials.Select Kopp’s rule for additive estimation of solid heat capacities. Verification / Alternative check: For salts like NaCl, summing Na and Cl atomic contributions via Kopp’s rule gives a reasonable first estimate compared with measured values, especially near room temperature. Why Other Options Are Wrong: Clapeyron: phase equilibrium slope; not heat capacity estimation. Gibbs: general thermodynamic relations; not an estimation rule here. Trouton: approximate entropy of vaporization near boiling point. Raoult: solution vapor pressure, unrelated to solid heat capacity. Common Pitfalls: Overextending Kopp’s rule beyond its validity range or expecting high-precision predictions. Always validate against data when available. Final Answer: Kopp’s rule

Question 11

Recycle ratio in process flows: Which expression correctly defines the recycle ratio used in chemical process calculations?

Accepted Answer

Introduction / Context: Recycle streams are common in reactors and separation systems to improve conversion, recover unreacted materials, and stabilize operation. The recycle ratio is a key dimensionless metric indicating how much material is being recirculated relative to the fresh input. Given Data / Assumptions: Definitions follow standard process design texts. Streams are clearly labeled: fresh feed, recycle, and gross (combined) feed. Concept / Approach: The recycle ratio quantifies the intensity of recirculation: it is most usefully defined relative to the fresh feed, since fresh feed sets the scale of new material entering the process. This helps analyze residence time, conversion, and equipment sizing impacts. Step-by-Step Solution: Let F_fresh be fresh feed rate, R be recycle rate, and F_gross = F_fresh + R.Recycle ratio (RR) = R / F_fresh.This definition allows easy interpretation: RR = 1 means as much material is recycled as is freshly fed. Verification / Alternative check: Process simulators and standard textbooks adopt RR = R / F_fresh. Other ratios can be defined for specific analyses but are not the conventional “recycle ratio.” Why Other Options Are Wrong: Gross/Fresh or Recycle/Gross are auxiliary ratios; they do not define the conventional recycle ratio. “None of these” is incorrect because a standard definition exists. Fresh/Recycle is the inverse and not the commonly reported metric. Common Pitfalls: Confusing gross feed with fresh feed, or using inconsistent bases when comparing cases. Always report which convention you use. Final Answer: Recycle stream / Fresh feed stream

Question 12

Autoionization of water and temperature effect: As the temperature of pure water increases, how do pH and pOH change?

Accepted Answer

Introduction / Context: Students often memorize pH = 7 for pure water, but this value applies strictly at 25 °C. The ionic product of water, Kw, is temperature dependent. Understanding how pH and pOH vary with temperature is essential for accurate acid–base calculations and water chemistry control. Given Data / Assumptions: Pure water, no added acid or base. As temperature increases above 25 °C, Kw increases. In pure water: [H+] = [OH−] = (Kw)^0.5 at any temperature. Concept / Approach: If Kw increases with temperature, both [H+] and [OH−] increase equally in pure water. Because pH = −log10[H+] and pOH = −log10[OH−], increases in concentrations lead to decreases in pH and pOH simultaneously. Neutrality still holds when [H+] = [OH−], but the neutral pH shifts below 7 as temperature rises. Step-by-Step Solution: At 25 °C: Kw ≈ 1.0e−14; pH_neutral = 7.00; pOH_neutral = 7.00.At higher T: Kw increases (e.g., ~5.5e−14 near 50 °C). Therefore [H+] and [OH−] both increase.Because pX = −log10[X], higher [H+] and [OH−] imply lower pH and lower pOH.Neutral condition persists (pH = pOH), but the neutral pH value is < 7. Verification / Alternative check: Pick Kw = 5.5e−14: [H+] = [OH−] = sqrt(5.5e−14) ≈ 7.4e−7 M; pH ≈ 6.13 and pOH ≈ 6.13. Both are less than 7, confirming the trend. Why Other Options Are Wrong: (a), (d) suggest one rises while the other falls, which contradicts equal increases in [H+] and [OH−]. (c) suggests both increase; they actually decrease as concentrations increase. (e) fixes pH at 7 regardless of temperature, which is false. Common Pitfalls: Believing “neutral = pH 7” at all temperatures; neglecting Kw’s temperature dependence in reactor, boiler, or cooling-water calculations. Final Answer: pOH and pH both decrease

Question 13

Colloid science fundamentals: What is the typical size range of colloidal particles (expressed in millimicrons or nanometers)?

Accepted Answer

Introduction / Context: Colloids are dispersions where the dispersed-phase particles are intermediate in size between true solutions and suspensions. Recognizing their size scale is crucial for predicting optical behavior (Tyndall effect), stability, and separation methods. Given Data / Assumptions: Classical colloid size range is typically cited as about 1 nm to 1000 nm (10 to 10,000 Å). Many textbooks emphasize a practical working range within these limits. Units: 1 millimicron = 1 nm; 1 Å = 0.1 nm. Concept / Approach: True solutions contain molecular or ionic species < 1 nm. Suspensions have particles generally > 1 μm (1000 nm). Colloids occupy the middle ground. In many practical contexts, 5–200 nm is often quoted as a representative colloidal size window for stability and optical effects in common systems. Step-by-Step Solution: Compare options with the colloid regime: nanometer scale fits, microns usually indicate suspensions.Option (a): 5–200 nm sits well within the commonly cited colloidal band.Option (d): 1–5 nm is at the very lower edge; many such species behave closer to large molecules/true solutions.Options (b), (c), and (e) are micrometer to millimeter scales, typical of suspensions, not colloids. Verification / Alternative check: Textbook ranges vary slightly, but the essence remains: colloids are between solution-sized species and coarse suspensions, with tens to hundreds of nanometers being archetypal. Why Other Options Are Wrong: (b), (c), (e): Particle sizes are too large; these are suspensions. (d): Too small and close to true solution sizes for most colloidal behaviors. Common Pitfalls: Confusing the borderline between large molecules and small colloids, or equating visibility under optical microscopes with colloidal classification. Always check the nanometer scale. Final Answer: 5 – 200 millimicrons (≈ 5 – 200 nm)

Question 14

Azeotropes and Raoult’s law deviations: An azeotropic mixture of two liquids exhibits a boiling point that is lower than the normal boiling point of either pure component when the solution shows which type of deviation from Raoult’s law?

Accepted Answer

Introduction / Context: Azeotropes are special liquid mixtures that boil at a constant temperature and composition. Distillation cannot separate their components beyond the azeotropic composition under the same pressure. Understanding how deviations from Raoult’s law (positive or negative) determine whether an azeotrope has a minimum or maximum boiling point is widely tested in chemical engineering thermodynamics and separation process design. Given Data / Assumptions: Binary, nonreactive liquid mixture. Equilibrium at a fixed total pressure (commonly 1 atm). Raoult’s law serves as the ideal-solution reference: p_i = x_i * p_i^sat. Positive deviation means weaker unlike-molecule attractions; negative deviation means stronger unlike interactions. Concept / Approach: When a mixture shows positive deviation from Raoult’s law, the escaping tendency (fugacity) of each component is higher than ideal. This raises the total vapor pressure above the ideal-solution prediction at a given temperature. Since boiling at fixed pressure occurs when total vapor pressure equals the external pressure, a higher total vapor pressure at the same composition implies the mixture reaches the boiling condition at a lower temperature than either pure component—hence a minimum-boiling (low-boiling) azeotrope. Conversely, negative deviation lowers total vapor pressure, yielding a maximum-boiling azeotrope. Step-by-Step Solution: Define deviation: positive deviation → p_total(actual) > p_total(ideal).At fixed P, higher p_total at a given T means the mixture will boil at a lower T than predicted for ideal behavior.If the boiling point falls below both pure-component boiling points at the azeotropic composition, a minimum-boiling azeotrope forms.Therefore, “lower than either” corresponds to positive deviation. Verification / Alternative check: Classical examples: ethanol–water (minimum-boiling azeotrope at ~95.6% ethanol by mass at 1 atm) exhibits positive deviation; hydrochloric acid–water (maximum-boiling) exhibits negative deviation. Why Other Options Are Wrong: Is saturated / Is unsaturated: these do not describe deviations from Raoult’s law and do not determine azeotrope type.Negative deviation: associated with maximum-boiling azeotropes (boiling point higher than either component). Common Pitfalls: Confusing minimum- and maximum-boiling azeotropes; assuming azeotropes always boil lower; ignoring pressure effects (changing pressure can create or eliminate azeotropes). Final Answer: shows positive deviation from Raoult's law

Question 15

Heat capacity additivity for solids: “The heat capacity of a solid compound is approximately equal to the sum of the atomic heat capacities of its constituent elements.” This empirical statement is known as:

Accepted Answer

Introduction / Context: Predicting the heat capacity of solid compounds is useful in calorimetry, thermal design, and materials selection. Kopp’s rule provides a quick estimate by assuming additivity of atomic heat capacities, while other named statements address different thermodynamic regularities. Given Data / Assumptions: Solid crystalline compound at moderate temperatures. Empirical approximation: compound heat capacity ≈ sum of atomic contributions. No strong phase transitions or anharmonic effects near the temperature of interest. Concept / Approach: Kopp’s rule extends the spirit of Dulong–Petit (which states that many elemental solids have molar heat capacity near 3R) to compounds by summing the atomic contributions. It is a first-cut estimate and may deviate when bonding, structure, or temperature-dependent effects are significant. In contrast, the Nernst heat theorem deals with entropy approaching zero as temperature approaches absolute zero, and Trouton’s rule relates normal boiling points to molar entropies of vaporization. Step-by-Step Solution: Identify the statement: “sum of the heat capacities of constituent elements.”Match to named rules: this is Kopp’s rule.Therefore, option (b) is correct. Verification / Alternative check: For many ionic solids (e.g., NaCl), summing cation and anion atomic heat capacities provides a reasonable estimate at ambient conditions; deviations grow at low T (quantum effects) and near transitions. Why Other Options Are Wrong: Dulong–Petit: applies to elemental solids, not directly an additivity rule for compounds.Nernst heat theorem: addresses entropy at T → 0 K.Trouton’s rule: correlates ΔS_vap at the normal boiling point; unrelated to solid heat capacities. Common Pitfalls: Overusing Kopp’s rule for precision design; confusing it with Dulong–Petit; ignoring strong temperature dependence. Final Answer: Kopp's rule

Question 16

Pressure equivalence and hydrostatic head: 1 kgf/cm^2 (kilogram-force per square centimetre) is approximately equal to which of the following?

Accepted Answer

Introduction / Context: Engineers frequently convert among distinct pressure units: kgf/cm^2, bar, kPa, psi, and hydrostatic head in metres of water (m H2O). Correct conversions are vital for sizing pumps, checking instrument ranges, and reading legacy plant data. Given Data / Assumptions: 1 kgf/cm^2 ≈ 0.9807 bar ≈ 98.07 kPa. Hydrostatic head of 10 m water at 4°C corresponds to ≈ 0.98 bar (density ~1000 kg/m^3, g ≈ 9.81 m/s^2). 1 atm ≈ 760 torr ≈ 1.01325 bar; 1 kPa is much smaller than 1 bar. Concept / Approach: Hydrostatic pressure P for a liquid column is P = rho * g * h. For water, rho*g ≈ 9810 Pa/m, hence 10 m water column ≈ 98.1 kPa ≈ 0.981 bar. Matching this to 1 kgf/cm^2 (≈ 98.1 kPa) confirms the conversion. This shows why older gauges labeled “kg/cm^2” align closely with “bar.” Step-by-Step Solution: Compute 1 kgf/cm^2 in SI: 1 kgf = 9.81 N; 1 cm^2 = 1e-4 m^2; so 1 kgf/cm^2 ≈ 9.81 / 1e-4 = 98,100 Pa = 98.1 kPa.Relate to water head: 98.1 kPa / (ρg) ≈ 98,100 / 9810 ≈ 10 m.Therefore 1 kgf/cm^2 ≈ 10 m of water column. Verification / Alternative check: Compare to atmospheres: 1 atm is 101.325 kPa, which corresponds to about 10.33 m of water—slightly higher than 10 m, mirroring the small difference between 1 kgf/cm^2 and 1 atm. Why Other Options Are Wrong: 760 torr corresponds to 1 atm, not 1 kgf/cm^2 (which is ~0.96 atm).1 kPa is far smaller; 1 kgf/cm^2 is ~98 kPa.1 m of water column is only ~9.81 kPa, one-tenth of the correct value. Common Pitfalls: Confusing kgf/cm^2 (force-based unit) with kg/cm^2 (mass per area); neglecting the slight differences between 1 bar, 1 atm, and 1 kgf/cm^2. Final Answer: 10 metres of water column

Question 17

Isotonic solutions and colligative properties: Two solutions are isotonic when they exhibit the same value of which property at the same temperature?

Accepted Answer

Introduction / Context: “Isotonic” solutions are fundamental in bioprocessing, medicine, and membrane science. The term refers to two solutions that exert the same osmotic effect across a semipermeable membrane at a given temperature, preventing net solvent flow. This concept is distinct from equality of concentration units like molarity or normality because solute dissociation and activity play major roles. Given Data / Assumptions: Comparison at the same temperature. Semipermeable membrane permeable to solvent but not solute. Possible electrolytes where van’t Hoff factor i > 1 due to dissociation. Concept / Approach: For dilute solutions, osmotic pressure follows the van’t Hoff relation: π = i C R T, where i is the van’t Hoff factor, C is molar concentration of solute particles, R is the gas constant, and T is absolute temperature. Two solutions are isotonic if their osmotic pressures are equal (π_1 = π_2). Equal molarity alone does not guarantee equal π when solutes differ in dissociation (different i) or when activity coefficients deviate from unity. Step-by-Step Solution: Define isotonic: equal osmotic pressure at the same temperature and across a given membrane.Relate to measurable quantities: π ∝ i C T; therefore equal π may arise from different C values depending on i.Conclude: the defining property is osmotic pressure, not viscosity, molarity, or critical temperature. Verification / Alternative check: Physiological saline (~0.9% NaCl) is isotonic to human blood plasma because its effective particle concentration (accounting for NaCl dissociation) yields an osmotic pressure similar to plasma—even though molar concentrations of individual species differ. Why Other Options Are Wrong: Viscosity: depends on interactions and size; not a colligative criterion.Molar concentration: ignores dissociation and activity effects.Critical temperature: a pure-component property, irrelevant to isotonicity. Common Pitfalls: Assuming equal normality equals isotonicity; forgetting electrolytes increase particle count via dissociation. Final Answer: osmotic pressure

Question 18

Identify the relation that is not an equation of state for real gases:

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Introduction / Context: Equations of state (EoS) relate P, V, and T of a fluid, enabling prediction of densities, fugacities, and phase behavior. Distinguishing EoS from empirical vapor-pressure correlations is critical when choosing models for design and simulation. Given Data / Assumptions: Real-gas behavior; need P–V–T relations for the bulk phase. Well-known EoS include virial, van der Waals, Redlich–Kwong, Peng–Robinson, etc. Antoine correlation gives p^sat(T) for a pure component; it is not a P–V–T relation. Concept / Approach: An equation of state must connect pressure, temperature, and molar (or specific) volume. The virial equation expands compressibility in powers of 1/V (or in Z). Van der Waals and Redlich–Kwong are cubic EoS directly providing P as a function of T and V. The Antoine equation, however, correlates saturation pressure with temperature only for a pure component and does not provide V; it is a property correlation, not an EoS. Step-by-Step Solution: List options: (a) virial → EoS; (b) van der Waals → EoS; (d) Redlich–Kwong → EoS; (c) Antoine → vapor-pressure correlation.Identify the odd one out: Antoine equation is not an equation of state. Verification / Alternative check: Software workflows use Antoine for bubble/dew calculations together with an EoS or activity model; this division of roles underscores that Antoine is not an EoS. Why Other Options Are Wrong: They are true equations of state connecting P–V–T for gases/fluids. Common Pitfalls: Confusing property correlations (Antoine) with EoS; assuming any pressure–temperature formula is an EoS. Final Answer: Antoine equation for vapor pressure: log10 p^sat = A − B/(C + T)

Question 19

Rigid vessel, saturated steam cooling: One kilogram of saturated steam at 100°C and 1.01325 bar is held in a rigid vessel of volume 1.673 m^3 and cooled to 98°C (saturation pressure 0.943 bar). At 98°C, one kilogram of saturated water vapor occupies…

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Introduction / Context: Steam in a rigid, closed vessel undergoes condensation when cooled at saturation. Because the container volume is fixed, part of the vapor condenses to meet the new saturation specific volume at the lower temperature. This classic thermodynamics exercise links mass–volume balance with latent heat magnitudes near 100°C. Given Data / Assumptions: Initial state: saturated vapor, T1 = 100°C, V_total = 1.673 m^3, m_total = 1 kg. Final state: saturated mixture at T2 = 98°C, p2 = 0.943 bar. Specific volume of saturated vapor at 98°C: v_g2 = 1.789 m^3/kg. Neglect vessel heat capacity and kinetic/potential energy effects. Concept / Approach: At the final saturated state, some vapor remains and the rest is condensate (liquid) occupying negligible volume compared with vapor near 100°C. Use the rigid-volume constraint to find vapor mass, then the condensed mass. The latent heat value near 100°C is about 2257–2260 kJ/kg; the problem tests consistency with typical steam-table values. Step-by-Step Solution: Let m_v be mass of vapor at 98°C. The vapor occupies nearly all the volume: m_v * v_g2 ≈ V_total.Compute m_v = 1.673 / 1.789 ≈ 0.936 kg.Condensed mass m_c = 1 − 0.936 ≈ 0.064 kg.Latent heat at ~100°C is close to 2260 kJ/kg; choices include 2263 kJ/kg, which matches standard values. Verification / Alternative check: Steam tables near 100°C give h_g − h_f ≈ 2256–2260 kJ/kg; at 98°C, the value is within a few kJ/kg of 2260. Hence 2263 kJ/kg is the appropriate selection. Why Other Options Are Wrong: 2676 kJ/kg: corresponds more to latent heat at lower pressures/high superheat contexts; not near 100°C saturation.40732 and 540 kJ/kg: orders of magnitude or phase-change type are incorrect; 540 is closer to water’s specific heat per kg raising 1 kJ/kg·K over 540 K, which is irrelevant here. Common Pitfalls: Forgetting the rigid-volume constraint; assigning significant volume to the small liquid fraction; mixing specific volume data across temperatures. Final Answer: 2263

Question 20

Units check — kinematic vs dynamic viscosity: Which of the following is not a unit of kinematic viscosity?

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Introduction / Context: Viscosity describes a fluid’s resistance to flow and appears in transport correlations and Reynolds-number evaluation. Two related properties exist: dynamic (absolute) viscosity μ and kinematic viscosity ν, related by ν = μ/ρ. Confusing their units can lead to incorrect dimensionless groups and erroneous designs. Given Data / Assumptions: Kinematic viscosity ν has SI units m^2/s; CGS unit is the stoke (1 St = 1 cm^2/s). Dynamic viscosity μ has SI units Pa·s; CGS unit is the poise (1 P = 1 g/(cm·s)). Concept / Approach: Identify which listed units represent ν. Since ν is an area per time, any unit equivalent to length^2/time qualifies. “Stoke” and “cm^2/s” are standard for kinematic viscosity in CGS. “Poise,” however, belongs to dynamic viscosity and therefore is not a kinematic viscosity unit. Step-by-Step Solution: Recall definitions: ν = μ/ρ; μ units differ from ν units.Map units: Poise → dynamic; Stoke and cm^2/s → kinematic.Select the option that is not a kinematic unit: Poise. Verification / Alternative check: 1 St = 100 cSt (centistokes); water at ~20°C has ν ≈ 1 cSt; μ for water is ≈ 1 cP (centipoise). The factor-of-100 scaling between cSt and cP reflects ρ ≈ 1 g/cm^3. Why Other Options Are Wrong: Stoke and cm^2/s are exactly kinematic viscosity units.“None of these” would imply all are units of ν, which is false. Common Pitfalls: Interchanging cP and cSt; forgetting density links μ and ν. Final Answer: Poise

Stoichiometry Questions