Solutions
What you must know
Understand solution formation, solubility rules, concentration units (molarity, mass percent, molality), dilution and mixing. Know factors affecting solubility: temperature, pressure (Henry's law for gases), and the like-dissolves-like principle. Understand colligative properties - vapour pressure lowering, boiling-point elevation, freezing-point depression, osmotic pressure - and their dependence on particle number. Know how electrolytes increase colligative effects relative to non-electrolytes.
Traps
- Confusing molarity (mol solute per dm³ solution) with molality (mol solute per kg solvent).
- Forgetting to account for the number of ions in colligative calculations.
- Assuming solubility always rises with temperature (gases decrease).
- Mixing volumes without considering volume change on mixing.
Worked example
Dilution: 50 cm³ of 2.0 mol dm⁻³ HCl diluted to 250 cm³. Moles = 0.050 × 2.0 = 0.100 mol; new concentration = 0.100/0.250 = 0.40 mol dm⁻³. Rule of thumb: moles of solute are conserved on dilution, so C₁V₁ = C₂V₂.