The pH Scale: A Mental Map
pH = −log[H⁺]. For strong acids (like HCl), [H⁺] = molarity directly. For weak acids, use Ka. For bases, find pOH first, then pH = 14 − pOH. Simple — but students mix these up constantly under pressure.
Strong vs Weak Acids
| Property | Strong Acid | Weak Acid |
|---|---|---|
| Dissociation | 100% | Partial (uses Ka) |
| pH formula | pH = −log C | pH = ½(pKa − log C) |
| Examples | HCl, H₂SO₄, HNO₃ | CH₃COOH, HCN, H₂CO₃ |
Buffer Solutions — The Henderson-Hasselbalch Equation
pH = pKa + log([A⁻]/[HA])
Where [A⁻] is the conjugate base (salt) concentration and [HA] is the weak acid concentration. Buffer capacity is maximum when pH = pKa (equal concentrations). A buffer resists pH change — it does NOT prevent pH change completely.
Solubility Product (Ksp)
Ksp = [M^n+]^a × [X^m−]^b for a sparingly soluble salt MₐXᵦ. Common Ion Effect: adding a common ion decreases solubility (shifts equilibrium left). Selective precipitation: use Ksp values to determine which salt precipitates first when adding an anion to a solution with two cations.
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