Choosing a concentration unit for solutions:\nIn routine chemical engineering calculations, which expression of solution concentration is generally the most convenient and broadly applicable?

Introduction / Context:
Different concentration units are used for different tasks. Selecting a convenient unit simplifies material balances, equilibrium calculations, and data correlation. For vapor–liquid equilibrium and many thermodynamic models, a composition variable that is dimensionless and independent of temperature and pressure is especially valuable.

Given Data / Assumptions:

• We seek a generally convenient unit across a range of operations and temperatures.
• No specific reaction stoichiometry that would favor equivalents (normality).
• Practical considerations such as temperature dependence (or lack thereof) matter.

Concept / Approach:
Mole fraction (x or y) is dimensionless, does not change with temperature or pressure (for a closed, nonreacting system), and is directly used in Raoult’s law, Dalton’s law, and activity/fugacity models. In contrast, molarity (mol/L) depends on temperature because solution volume changes. Molality (mol/kg solvent) is temperature independent and excellent for colligative properties, but it is less directly used in gas–liquid equilibrium expressions. Normality (equivalents/L) is reaction-specific and can be ambiguous unless the reaction is specified.

Step-by-Step Solution:
Assess each unit for general convenience across thermodynamics and separations.Note that mole fraction integrates seamlessly with Raoult’s and Dalton’s laws in equilibrium work.Conclude that “mole fraction” is the most broadly convenient choice here.

Verification / Alternative check:
Most VLE and EoS-based computations use mole fractions as primary composition variables; software packages likewise adopt x and y for liquid and vapor phases.

Why Other Options Are Wrong:
Normality: depends on reaction equivalence; not universal.Molality: useful and temperature independent, but less convenient for gas-phase calculations and phase-rule applications.Molarity: varies with temperature because volume changes, complicating wide-range calculations.

Common Pitfalls:
Interchanging molarity and molality; attempting to use normality without specifying the reaction; overlooking temperature effects on molarity.

Final Answer:
Mole fraction