Identify the incorrect statements: Which of the following statements are wrong regarding colligative properties and concentration definitions?
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AEvaporation of an aqueous glucose solution increases its molarity
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BBoth the freezing point and the boiling point of sea water are higher than those of distilled water
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CA solution prepared with equal masses of two different liquids X and Y has equal mole fractions of X and Y
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DStatements (b) and (c) are wrong
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EAll statements (a), (b), and (c) are wrong
Answer
Correct Answer: Statements (b) and (c) are wrong
Explanation
Introduction / Context:This question probes understanding of molarity changes upon evaporation and the effects of dissolved salts on freezing and boiling points, as well as the distinction between mass fraction and mole fraction. Careful reasoning about definitions and colligative properties is essential in solution chemistry and process calculations.
Given Data / Assumptions:
- Glucose is nonvolatile and non-dissociating in water.
- Sea water contains salts (e.g., NaCl, MgSO4) increasing total solute concentration.
- Liquids X and Y are different substances with generally different molar masses.
Concept / Approach:For statement (a): Molarity = moles of solute per litre of solution. Evaporation removes solvent, decreasing solution volume while moles of solute remain constant; hence molarity increases. For statement (b): Dissolved salts lower freezing point and raise boiling point relative to pure water; therefore, sea water has a lower freezing point and a higher boiling point, not both higher. For statement (c): Equal masses do not generally imply equal moles unless molar masses are identical; therefore, mole fractions are generally unequal.
Step-by-Step Solution:
Check (a): Volume decreases on evaporation; molarity increases → (a) is correct.Check (b): Freezing point depression makes T_f lower; boiling point elevation makes T_b higher → (b) is wrong.Check (c): n = m/M; equal masses with different M give different moles → (c) is wrong.Therefore, the wrong statements are (b) and (c).Verification / Alternative check:Colligative property formulas: ΔT_b = K_b * m and ΔT_f = K_f * m confirm qualitative trends for electrolytes in sea water.
Why Other Options Are Wrong:
- Option (a): correctly states molarity increases.
- Option (e): not all statements are wrong; (a) is correct.
Common Pitfalls:Confusing molarity (volume-based) with molality (mass of solvent-based). Evaporation does not change molality when only solvent is lost, but it does increase molarity.
Final Answer:Statements (b) and (c) are wrong.