Questions: Colligative Properties: Effects of Solute Concentration

Question 1

A chemist dissolves 1 mole of glucose (C₆H₁₂O₆) and 1 mole of NaCl in separate beakers of water. Which solution shows a greater freezing point depression?

• A. The glucose solution, because it has more atoms per molecule contributing to the effect
• B. Both are equal, since the same number of moles was dissolved
• C. The NaCl solution, because it dissociates into two particles (Na⁺ and Cl⁻) per formula unit
• D. The glucose solution, because ionic compounds like NaCl have weaker colligative effects due to ion pairing

Answer: (C)

Colligative properties depend on the number of dissolved particles, not chemical identity. NaCl dissociates into ~2 particles per formula unit (van't Hoff factor i ≈ 2), roughly doubling the effective particle concentration compared to glucose, which remains as intact molecules (i = 1). This is the core of colligative reasoning: identical mole amounts of different solutes can have very different effects if one ionizes.

Question 2

Which fundamental phenomenon is the ROOT cause underlying boiling point elevation, freezing point depression, and osmotic pressure?

• A. Increased hydrogen bonding between solute and solvent molecules
• B. Vapor pressure lowering — solute particles occupy surface sites, reducing solvent escape into the gas phase
• C. Increased ionic strength that disrupts solvent molecule interactions
• D. Increased heat capacity of the solution

Answer: (B)

Vapor pressure lowering is the underlying mechanism from which all other colligative properties cascade. When solute particles occupy surface positions, fewer solvent molecules escape into the gas phase, lowering vapor pressure. A liquid boils when its vapor pressure equals atmospheric pressure — so lower vapor pressure requires higher temperature (boiling point elevation). Freezing point depression and osmotic pressure arise from the same thermodynamic root.

Question 3

A 1 molal solution of CaCl₂ produces a greater boiling point elevation than a 1 molal solution of NaCl.

• T. True
• F. False

Answer: True

CaCl₂ dissociates into three ions per formula unit (Ca²⁺ + 2 Cl⁻, i ≈ 3), while NaCl produces two ions (i ≈ 2). Since ΔTb = Kb × m × i, the CaCl₂ solution has a higher effective particle concentration and a larger boiling point elevation. This is why the van't Hoff factor is crucial: failing to account for ionization leads to underestimating the colligative effect of electrolytes.

Question 4

Because colligative properties depend primarily on particle count, a 1 molal solution of any strong electrolyte will generally produce exactly the colligative effect predicted by multiplying its ideal van't Hoff factor by the molal constant.

• T. True
• F. False

Answer: False

In concentrated solutions, opposite-charged ions can associate transiently into ion pairs, reducing the effective number of independent particles below the ideal integer value. A 1 molal NaCl solution behaves as if i ≈ 1.9 rather than exactly 2. The van't Hoff factor is only truly 'ideal' at infinite dilution; at realistic concentrations, ion pairing makes the actual colligative effect somewhat smaller than the simple calculation predicts.

Question 5

Why is osmotic pressure preferred over boiling point elevation for determining the molar mass of large molecules like proteins?

Model answer: Proteins have very high molar masses (often 10,000–500,000 g/mol), so even dissolving several grams produces very few moles — resulting in extremely low molality. The boiling point elevation ΔTb = Kb × m would be immeasurably tiny (often < 0.001°C). Osmotic pressure π = iMRT is far more sensitive: even a dilute solution generates a measurable pressure difference across a semipermeable membrane, making it practical to determine molar mass at the concentrations used in biochemical experiments.

Sensitivity scales differently across colligative properties at very low concentrations. Osmotic pressure is linear in molarity and involves RT (~2.5 kJ/mol at room temperature), producing pressures easily measured with a manometer even for micromolar solutions. This makes it the tool of choice in biochemistry for molar mass determination of macromolecules.