Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A salt’s solubility depends on pH when one of its ions is a weak base, a weak acid or OH⁻.
- Acid (H₃O⁺) removes a basic anion by turning it into its conjugate acid. Base (OH⁻) removes a weakly acidic cation by turning it into its conjugate base.
- Removing an ion makes Q < K_sp, so the equilibrium shifts towards the dissolved ions and more solid dissolves.
- K_sp does not change. Only the solubility changes.
- The course tests this qualitatively. You will not calculate solubility as a function of pH.
Key relationships
| Type of salt | Examples | In acid | In base |
|---|---|---|---|
| Anion is a weak base | CaF₂, CaCO₃, Ca₃(PO₄)₂ | more soluble | about the same, or slightly less |
| Hydroxide | Mg(OH)₂, Fe(OH)₃ | more soluble | less soluble (OH⁻ is a common ion) |
| Cation is a weak acid (BH⁺) | fictional (BH)X | about the same | more soluble |
| Anion of a strong acid, neutral cation | AgCl, AgBr, AgI | no change | no change |
Useful equations: F⁻ + H₃O⁺ → HF + H₂O; OH⁻ + H₃O⁺ → 2 H₂O; CO₃²⁻ + 2 H₃O⁺ → CO₂ + 3 H₂O; BH⁺ + OH⁻ → B + H₂O.
pH versus pK_a. A basic anion A⁻ is mostly removed once the pH is below the pK_a of HA. A cation BH⁺ is mostly removed once the pH is above the pK_a of BH⁺.
Assumptions behind the reasoning
- Temperature is constant, so K_sp is constant.
- The added acid or base does not supply a common ion (HCl would add Cl⁻; NaOH adds OH⁻).
- In experiments, a buffer holds the pH steady and its ions do not react with the salt.
Mistakes to avoid
- Saying K_sp increases in acid. Say the solubility increases.
- Making every salt pH-sensitive. Check the ions first: Cl⁻, Br⁻ and I⁻ do not react with H₃O⁺.
- Reacting the acid with the metal ion. The proton goes to the basic anion or to OH⁻.
- Forgetting the common ion. NaOH lowers the solubility of Mg(OH)₂; HCl lowers the solubility of a chloride.
- Skipping the Q step. A full explanation says which ion is removed, that Q falls below K_sp, and which way the equilibrium shifts.
- Claiming a trend inside the error. Compare differences in the data with the precision of the balance.
Quick self-check
- Does BaCO₃ dissolve more in HNO₃(aq) or in water? (In HNO₃: H₃O⁺ removes CO₃²⁻)
- Does adding acid change the solubility of AgI? (No: I⁻ is the conjugate base of a strong acid)
- What does adding NaOH do to the solubility of Fe(OH)₃? (Lowers it: OH⁻ is a common ion)
- Which way does the solubility of a salt with a weakly acidic cation change as the pH rises? (It increases)
Next: practice questions.