Why is molality preferred over molarity in colligative-property calculations?

Molality is mass-based and does not change with temperature. Colligative formulae require a temperature-independent measure of particle concentration in the solvent.

What is the van't Hoff factor i?

The ratio of actual particle count to formula-unit count. For a strong electrolyte like NaCl, i ≈ 2; for weak electrolytes it lies between 1 and the maximum dissociation value.

What is an azeotrope?

A liquid mixture that boils at a constant temperature with the same composition in vapour and liquid; it cannot be separated by simple distillation. 95.6% ethanol-water is the classic example.

Why is osmotic pressure the preferred colligative property for proteins?

Proteins have very high molar mass, so freezing-point depression and boiling-point elevation are too small to measure accurately. π remains measurable even at very low molar concentration.

Does dissolving a solute always lower vapour pressure?

Only if the solute is non-volatile. A volatile solute contributes its own vapour pressure, so total vapour pressure may rise or fall depending on Raoult's law.

Which property is NOT colligative?

Density of solution. Density depends on the identity of the solute. Colligative properties depend only on particle count.

0.5 mol of glucose dissolved in 1 kg water raises the boiling point by (K_b = 0.512):

0.256 °C. ΔT_b = K_b × m × i = 0.512 × 0.5 × 1 = 0.256 °C (i = 1 for non-electrolyte).

The osmotic pressure of a 0.10 M sucrose solution at 27 °C is approximately:

2.46 atm. π = MRT = 0.10 × 0.0821 × 300 = 2.46 atm.

Which aqueous solution will have the lowest freezing point at the same molality?

AlCl₃. ΔT_f scales with i. AlCl₃ gives i ≈ 4 (Al³⁺ + 3 Cl⁻), the largest among these.

A solution shows positive deviation from Raoult's law when:

A-B forces are weaker than A-A and B-B. Weaker A-B interactions allow molecules to escape more easily, raising vapour pressure above Raoult prediction.

Solutions and Colligative Properties MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Solutions and Colligative Properties

Solutions and Colligative Properties averages 2 MCQs per MDCAT paper — molality calculations, freezing-point depression, and Raoult's law are most tested.

Solutions and Colligative Properties is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

Types of solutions and concentration units

A solution is a homogeneous mixture of a solute (lesser) in a solvent (greater); both can be solid, liquid, or gas. Concentration units used in colligative work: molality m = mol solute / kg solvent (preferred — temperature independent), molarity M = mol solute / L solution, mole fraction x = n_solute/n_total, mass percent. Atkins Chapter 5 stresses that converting between molality and molarity requires the solution density. For dilute aqueous solutions at 25 °C, molality and molarity are numerically very close because density ≈ 1 g/mL and solute mass is small.

Raoult's law and ideal solutions

For an ideal solution of two volatile liquids: P_A = x_A^liq P_A°, P_B = x_B^liq P_B°, total P = P_A + P_B. This is Raoult's law. Mixtures obeying it (benzene-toluene, n-hexane-n-heptane) form ideal solutions. Positive deviations (acetone-CS₂) occur when A-B forces are weaker than A-A or B-B; negative deviations (acetone-chloroform with H-bonding) occur when A-B forces are stronger. Azeotropes form at composition extremes of strong deviation — they cannot be separated by simple distillation.

Lowering of vapour pressure and boiling point elevation

A non-volatile solute lowers the solvent's vapour pressure: ΔP = x_solute P°_solvent. This raises the boiling point: ΔT_b = K_b · m, where K_b for water is 0.512 °C/m. Adding 1 mol of glucose to 1 kg of water raises its boiling point by 0.512 °C. For ionic solutes, multiply by the van't Hoff factor i (number of particles per formula unit): NaCl gives i = 2, so 1 m NaCl raises T_b by ~1.02 °C. FSc XI Chapter 9 tabulates K_b and K_f for common solvents.

Freezing point depression and osmotic pressure

ΔT_f = K_f · m · i; for water K_f = 1.86 °C/m. This is why salt is spread on icy roads (lowers freezing point of water) and why ethylene glycol is added to car radiators (50% v/v glycol lowers T_f to about −37 °C). Osmotic pressure π = MRT (van't Hoff equation); a 0.1 M solution at 298 K exerts π = 0.1 × 0.0821 × 298 ≈ 2.45 atm. Reverse osmosis (applying P > π) is the basis of desalination. All four colligative properties depend only on the number of solute particles, not their identity — the defining feature.

Determining molar mass from colligative data

Dissolving 1.20 g of a non-electrolyte in 50.0 g of water depresses the freezing point by 0.40 °C: m = ΔT_f/K_f = 0.40/1.86 = 0.215 mol/kg, so moles in 50 g water = 0.0108, M = 1.20/0.0108 ≈ 111 g/mol. Osmotic pressure works similarly and is preferred for macromolecules (proteins, polymers) because π is measurable even when mass concentration is tiny — vital in protein chemistry, as Atkins highlights.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Solutions and Colligative Properties Is Tested

MDCAT questions on Solutions and Colligative Properties are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.