U7.13 · pH and SolubilityPractice · pH 与溶解度

When the anion is a base (OH⁻) or the conjugate base of a weak acid (CO₃²⁻, PO₄³⁻), H⁺ combines with it, lowering its concentration so Q < Ksp and more salt dissolves. I⁻ and Br⁻ are conjugate bases of the strong acids HI and HBr, so they barely combine with H⁺, and those salts’ solubility hardly depends on pH.

pH = 3.00, so pOH = 11.00 and [OH⁻] = 1 × 10⁻¹¹ M, held fixed by the buffer. [Fe³⁺] = s = Ksp ÷ [OH⁻]³ = 4 × 10⁻³⁸ ÷ (1 × 10⁻¹¹)³ = 4 × 10⁻⁵ M. It is almost insoluble in neutral water, but the more acidic the solution, the more dissolves, which is why acid removes rust.

CaCO₃(s) ⇌ Ca²⁺ + CO₃²⁻. CO₃²⁻ is the conjugate base of a weak acid, so the H⁺ from acetic acid uses it up and the equilibrium keeps shifting toward dissolving, while CO₂ escapes. Acetic acid is weak, calcium acetate is soluble, and Ksp changes only with temperature.

Acid in the mouth (H⁺) combines with the OH⁻ and PO₄³⁻ in hydroxyapatite, dissolving enamel. F⁻ is the conjugate base of HF and far less basic than OH⁻, so H⁺ hardly combines with it, and fluorapatite dissolves less in acid. F⁻ doesn’t neutralize acid, and fluorapatite still contains phosphate.

Add Mg(OH)₂(s) ⇌ Mg²⁺ + 2OH⁻ (Ksp) and 2 × [H⁺ + OH⁻ ⇌ H₂O] (each with K = 1/Kw); the 2OH⁻ cancel. K = Ksp × (1/Kw)² = 8.9 × 10⁻¹² ÷ (1.0 × 10⁻¹⁴)² = 8.9 × 10¹⁶. K is huge, so Mg(OH)₂ dissolves readily in acid, which is how it works as an antacid.

HF is a weak acid (Ka ≈ 6.4 × 10⁻⁴), so when the pH approaches or drops below its pKa (about 3.2), most F⁻ is captured by H⁺ as HF. Lower [F⁻] shifts CaF₂(s) ⇌ Ca²⁺ + 2F⁻ to the right, so the solubility rises: about 6.5 times its pH 7 value at pH 2. HCl is a strong acid, so Cl⁻ hardly combines with H⁺ and AgCl’s solubility doesn’t change with pH. Ca²⁺ doesn’t react with H⁺, and AgCl’s Ksp is actually much smaller than CaF₂’s.