Solutions
Solutions tests how solute particles change vapour pressure, boiling point, freezing point and osmotic pressure, and how real solutions depart from ideal behaviour. Numerical questions on colligative properties and van't Hoff factor are common.
Core rules
- Raoult's law for volatile components: p_A = x_A p_A^0; total p = p_A + p_B.
- Colligative properties depend on particle number: ΔTb = i Kb m, ΔTf = i Kf m, π = iCRT.
- Ideal solutions: ΔHmix = 0, ΔVmix = 0, obey Raoult at all compositions. Non-ideal: positive deviation (ethanol-acetone) or negative deviation (chloroform-acetone).
- van't Hoff factor i > 1 for dissociation (NaCl i ≈ 2), i < 1 for association (benzoic acid in benzene).
Worked example
Find osmotic pressure of 0.10 M NaCl at 300 K. NaCl dissociates into two ions, so i = 2. π = iCRT = 2 x 0.10 x 0.0821 x 300 = 4.93 atm. Reasoning: osmotic pressure counts particles, so the dissociation factor must multiply the formal concentration.
Traps
Using molarity where molality is required in ΔTb and ΔTf, forgetting i, and assuming i is exactly 2 when ion pairing reduces it. In positive deviations, solute-solvent forces are weaker than in pure components, so vapour pressure is higher than Raoult's law predicts.