Solutions
Solutions tests concentration units, Raoult's law, colligative properties and deviations from ideality. Numerical questions often combine molality, van't Hoff factor and molar mass.
Concentration and Raoult
Molarity changes with temperature; molality and mole fraction do not. Raoult's law for a volatile solvent is p = x_solvent p0. Ideal solutions have delta H = 0 and delta V = 0. Positive deviation means A-B forces are weaker than A-A/B-B (ethanol-water); negative deviation means stronger (HNO3-water). Minimum-boiling azeotropes show positive deviation; maximum-boiling azeotropes show negative deviation.
Colligative properties
Relative lowering of vapour pressure equals mole fraction of solute. Boiling-point elevation is delta Tb = i Kb m; freezing-point depression is delta Tf = i Kf m; osmotic pressure is pi = i C R T. The van't Hoff factor i = observed colligative effect/calculated effect = normal molar mass/observed molar mass. Dissociation gives i > 1; association gives i < 1.
Worked example
1.0 g urea (M = 60) in 100 g water: moles = 0.0167, molality = 0.0167/0.100 = 0.167 mol/kg. With Kf = 1.86 K kg/mol, delta Tf = 1.86 x 0.167 = 0.31 K. If NaCl were used at the same molality, i = 2 and delta Tf = 0.62 K, because each formula unit gives two ions.
Traps
Dividing by grams of solution instead of kg of solvent, forgetting i for electrolytes, and using molarity in Kf expressions are common errors.