Solutions and Electrolytic Dissociation
Core concepts
Understand concentration (molarity, mass percent, molality), dilution, solubility rules, and the behaviour of strong and weak electrolytes. Strong electrolytes dissociate completely (strong acids, strong bases, soluble salts); weak electrolytes do so partially (weak acids, weak bases, water). Know the Arrhenius and Brønsted–Lowry definitions of acids and bases, the ionic product of water, pH and pOH, and the idea of hydrolysis of salts.
Traps that appear in questions
- Dilution: moles of solute stay constant; use c₁V₁ = c₂V₂ and remember the volume is the final total volume.
- Strong vs weak: pH calculations for weak acids require the dissociation constant; do not use the full concentration.
- Salt hydrolysis: salts of strong acid and strong base are neutral; strong acid with weak base give acidic solutions; weak acid with strong base give basic solutions.
- Precipitation: apply the solubility rules before writing products; a precipitate is only written if it is insoluble.
- Electrical conductivity: it depends on the number of mobile ions, not on the concentration of undissociated molecules.
Worked example
What is the pH of 0.01 mol/L hydrochloric acid? HCl is strong, so [H⁺] = 0.01 = 10⁻² mol/L, and pH = −log(10⁻²) = 2. For 0.01 mol/L acetic acid, the pH is higher (about 3.4) because the acid dissociates only partially.
Rule of thumb
Classify the electrolyte first: strong electrolytes give full ion concentrations, weak ones need an equilibrium calculation.