\[\text{强碱弱酸盐：}[\mathrm{OH^-}] = \sqrt{\frac{K_w}{K_a}c};\qquad \text{强酸弱碱盐：}[\mathrm H^+] = \sqrt{\frac{K_w}{K_b}c};\qquad \text{弱酸弱碱盐：}[\mathrm H^+] \approx \sqrt{\frac{K_wK_a}{K_b}}\]\[\text{weak-acid salt of strong base :}[\mathrm{OH^-}] = \sqrt{\frac{K_w}{K_a}c};\qquad \text{weak-base salt of strong acid :}[\mathrm H^+] = \sqrt{\frac{K_w}{K_b}c};\qquad \text{weak acid–weak base salt :}[\mathrm H^+] \approx \sqrt{\frac{K_wK_a}{K_b}}\]
含义Meaning
弱酸的阴离子（或弱碱的阳离子）与水反应使溶液呈碱性（酸性），水解常数 \(K_h = K_w/K_a\)（或 \(K_w/K_b\)）。
Anions of weak acids (cations of weak bases) react with water, making solutions basic (acidic); the hydrolysis constant is Kw/Ka (or Kw/Kb).
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(K_h\) | 水解常数hydrolysis constant | 1 |
| \(c\) | 盐浓度salt concentration | mol·L⁻¹ |
推导Derivation
A⁻ + H₂O ⇌ HA + OH⁻ 就是共轭碱的碱式解离，\(K_b(\mathrm A^-) = K_w/K_a\)。
A⁻ hydrolysis is the base reaction of A⁻ with Kb = Kw/Ka.
用弱碱公式即得。
Apply the weak-base formula.
弱酸弱碱盐（如 NH₄Ac）阴阳离子同时水解，近似 pH = ½(pKw + pKa − pKb)，与浓度无关；Ka = Kb 时为中性。
For salts of a weak acid and weak base, pH ≈ ½(pKw + pKa − pKb), independent of c.
适用条件与局限Assumptions & limitations
多价金属离子（Fe³⁺、Al³⁺）水解生成多核羟基配合物，需用逐级水解常数。
Multivalent metal ions form polynuclear hydroxo complexes; use stepwise constants.
例题Worked example
0.10 mol/L CH₃COONa：[OH⁻] = √(10⁻¹⁴/1.8×10⁻⁵ × 0.10) = 7.5×10⁻⁶，pH = 8.87。
0.10 M sodium acetate: pH = 8.87.