Solutions and Colligative Properties
Solutions and Colligative Properties
This domain tests concentration, solubility and colligative properties: vapour pressure lowering, boiling point elevation, freezing point depression and osmotic pressure. These properties depend on the number of dissolved particles, not their identity.
Core rules
- Molarity = moles solute per litre of solution; molality = moles solute per kilogram of solvent.
- Mole fraction = moles of component / total moles.
- Raoult's law for a non-volatile solute: vapour pressure of solution = mole fraction of solvent × vapour pressure of pure solvent.
- Boiling point elevation: ΔTb = iKb m. Freezing point depression: ΔTf = iKf m.
- Osmotic pressure: π = iMRT.
- The van't Hoff factor i is the number of particles per formula unit: 1 for glucose, 2 for NaCl, 3 for CaCl2 (assuming complete dissociation).
Worked example
5.00 g NaCl (M = 58.5 g mol^-1) is dissolved in 100 g water. n(NaCl) = 5.00/58.5 = 0.0855 mol; molality = 0.0855/0.100 = 0.855 mol kg^-1. For NaCl, i = 2. ΔTf = iKf m = 2 × 1.86 × 0.855 = 3.18 °C. The solution freezes at about −3.18 °C.
Traps
Use molality, not molarity, in colligative formulas. Include i for electrolytes. Colligative properties assume a non-volatile solute and ideally dilute solution. Do not add the mass of solute to solvent mass when calculating molality unless the question defines the solution mass.