Acids, Bases & pH
Acid-base models, pH/pOH, conjugate pairs, strong vs weak, Ka/Kb, salt hydrolysis, and polyprotic acids.
Three models
REFERENCE| Model | Acid | Base |
|---|---|---|
| Arrhenius | makes H⁺ in water | makes OH⁻ in water |
| Brønsted-Lowry | donates H⁺ | accepts H⁺ |
| Lewis | accepts an e⁻ pair | donates an e⁻ pair |
pH, pOH, Kw
CORE RULEKw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00. [H⁺] = 10^(−pH). pH 7 neutral, <7 acidic, >7 basic.
Conjugate pairs
CORE RULEA conjugate pair differs by one H⁺. In HA + B ⇌ A⁻ + BH⁺: (HA, A⁻) and (B, BH⁺). Amphiprotic species (H₂O, HCO₃⁻, H₂PO₄⁻) act as acid or base depending on the partner.
Strong acids & bases
CORE RULEIonize completely. Strong acids: HCl, HBr, HI, HNO₃, HClO₃, HClO₄, H₂SO₄ (1st only). Strong bases: group 1 hydroxides; Ca/Sr/Ba(OH)₂ (2 OH⁻). Monoprotic: [H⁺] = [HA]₀. n-OH base: [OH⁻] = n[B]₀.
Weak acid (Ka)
CORE RULEHA ⇌ H₃O⁺ + A⁻, Ka = [H₃O⁺][A⁻]/[HA]. ICE gives x²/([HA]₀ − x) = Ka. If 100·Ka < [HA]₀, x ≈ √(Ka·[HA]₀); pH = −log x. Otherwise use the quadratic.
Weak base (Kb)
CORE RULEB + H₂O ⇌ BH⁺ + OH⁻, Kb = [BH⁺][OH⁻]/[B]. ICE: x²/([B]₀ − x) = Kb; if 100·Kb < [B]₀, x ≈ √(Kb·[B]₀); [OH⁻] = x, pOH = −log x, pH = 14 − pOH.
Ka · Kb = Kw
CORE RULEFor a conjugate pair: Ka·Kb = Kw = 1.0 × 10⁻¹⁴, so pKa + pKb = 14. Stronger acid → weaker conjugate base. Get a salt anion's Kb = Kw/Ka, or a cation's Ka = Kw/Kb.
Salt hydrolysis
REFERENCE| Salt (from) | Solution |
|---|---|
| strong acid + strong base (NaCl) | neutral |
| strong acid + weak base (NH₄Cl) | acidic |
| weak acid + strong base (NaCH₃COO) | basic |
| weak acid + weak base | compare Ka vs Kb |
Percent ionization
CORE RULE% ionization = (x / [HA]₀) × 100, x from the ICE table. Larger Ka → more ionized; more dilute → more ionized (Le Chatelier on dilution).
Polyprotic acids
CORE RULEStepwise: Ka1 ≫ Ka2 ≫ Ka3 (by ~10⁴-10⁵). When Ka1 ≥ 20·Ka2, treat as monoprotic for pH (Ka1 ICE alone). Then [X²⁻] ≈ Ka2, independent of concentration. E.g. H₃PO₄: Ka1 7.5 × 10⁻³, Ka2 6.2 × 10⁻⁸, Ka3 4.8 × 10⁻¹³.