Solutions
Unit 9 tests concentration units, Raoult's law, deviations from ideality and the four colligative properties.
Core rules
- Concentration units: molarity (mol L⁻¹ of solution), molality (mol kg⁻¹ of solvent), mole fraction, ppm and normality.
- Raoult's law: partial vapour pressure of a volatile component = its mole fraction × pure vapour pressure. For a non-volatile solute, relative lowering of vapour pressure equals the mole fraction of solute.
- Positive deviations (ethanol–acetone) arise when A–B forces are weaker than A–A and B–B; negative deviations (acetone–chloroform) arise when they are stronger. Azeotropes boil at constant composition.
- Colligative properties: ΔTb = Kb·m, ΔTf = Kf·m, π = CRT. With an electrolyte, multiply by the van't Hoff factor i.
- Molar mass from colligative data is abnormal when association (i < 1) or dissociation (i > 1) occurs.
Worked example
1.8 g of a non-volatile solute in 100 g of water depresses the freezing point by 0.186 K. With Kf = 1.86 K kg mol⁻¹, m = ΔTf/Kf = 0.186/1.86 = 0.100 mol kg⁻¹. Moles in 0.100 kg of water = 0.0100 mol. Molar mass = 1.8/0.0100 = 180 g mol⁻¹.
Traps
- Molality uses solvent mass in kg and does not change with temperature; molarity does.
- For NaCl, i ≈ 2; for glucose, i = 1; for acetic acid in benzene, i < 1 because of dimerisation.
- Raoult's law applies to volatile components in the mixture.
- Osmotic pressure is measured in atm or Pa and uses T in kelvin.
- Check whether the solute is an electrolyte before using the simple formula.