Solutions, Acids and Bases
The Solutions, Acids and Bases domain tests concentration, solubility, colligative properties, pH, strong versus weak acids and bases, buffers, and titration. Expect items that require calculating pH or concentration and interpreting equilibrium in aqueous systems.
Core rules and concepts
- Molarity M = moles solute / liters solution; dilution uses M₁V₁ = M₂V₂ because moles solute stay constant.
- Solubility depends on solute-solvent interactions and temperature; “like dissolves like” is a useful guide.
- Colligative properties depend on particle count: vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmosis.
- pH = −log[H₃O⁺]; pOH = −log[OH⁻]; at 25 °C, pH + pOH = 14 and Kw = 1.0 × 10⁻¹⁴.
- Strong acids and bases dissociate completely; weak acids and bases reach equilibrium described by Ka or Kb.
- Buffers resist pH change and are most effective when pH is near pKa.
Worked example
Find the pH of 0.010 M acetic acid, Ka = 1.8 × 10⁻⁵. Let x = [H₃O⁺]. Ka = x² / (0.010 − x). Because Ka is small, assume x << 0.010, so x² ≈ (1.8 × 10⁻⁵)(0.010) = 1.8 × 10⁻⁷, giving x = 4.2 × 10⁻⁴ M. pH = −log(4.2 × 10⁻⁴) = 3.37. The assumption is reasonable because x is about 4% of 0.010 M.
Traps that cost marks
- Setting [H₃O⁺] equal to the acid molarity for a weak acid.
- Using pH + pOH = 14 at temperatures other than 25 °C.
- Confusing dilution with a change in total moles of solute.
- Forgetting that strong acids fully ionize while weak acids do not.
- Expecting a buffer to hold pH constant regardless of added amount.