Salt hydrolysis happens when ions from a dissolved salt react with water to change the concentration of H3O+ or OH− in solution. This is why some salts make water acidic, some make it basic, and some leave it nearly neutral. The idea matters because it connects acid base theory, equilibrium, buffers, and pH prediction.
It also helps explain everyday and laboratory solutions such as ammonium chloride, sodium acetate, and sodium chloride.
Key Facts
- Salt hydrolysis is the reaction of a salt ion with water to produce H3O+ or OH−.
- A salt made from a strong acid and a strong base is usually neutral, such as NaCl.
- A salt made from a strong acid and a weak base is acidic, such as NH4Cl.
- A salt made from a weak acid and a strong base is basic, such as CH3COONa.
- For the conjugate base of a weak acid, Kb = Kw / Ka.
- For the conjugate acid of a weak base, Ka = Kw / Kb.
Vocabulary
- Salt hydrolysis
- Salt hydrolysis is the reaction between an ion from a dissolved salt and water that can change the pH of the solution.
- Conjugate acid
- A conjugate acid is the particle formed when a base gains a proton.
- Conjugate base
- A conjugate base is the particle formed when an acid loses a proton.
- Spectator ion
- A spectator ion is an ion that stays dissolved but does not significantly react with water or affect pH.
- Hydronium ion
- The hydronium ion, H3O+, is formed when water accepts a proton and is responsible for acidity in aqueous solution.
Common Mistakes to Avoid
- Assuming every salt solution is neutral is wrong because ions from weak acids or weak bases can react with water and shift the pH.
- Treating Na+ or Cl− as hydrolyzing ions is wrong because they come from the strong base NaOH and strong acid HCl, so they are usually spectators in water.
- Forgetting to identify the parent acid and base is wrong because the strength of those parents determines whether each ion is acidic, basic, or neutral.
- Using Ka instead of Kb for a conjugate base is wrong because a basic anion reacts with water to form OH−, so its equilibrium is described by Kb = Kw / Ka.
Practice Questions
- 1 Predict whether 0.10 M NH4Cl is acidic, basic, or neutral. Write the hydrolysis equation for the ion that reacts with water.
- 2 Sodium acetate, CH3COONa, has acetate ion with Ka for acetic acid equal to 1.8 × 10−5. Calculate Kb for CH3COO− using Kw = 1.0 × 10−14.
- 3 A salt contains one cation that is the conjugate acid of a weak base and one anion that is the conjugate base of a weak acid. Explain what information you need to predict whether the solution is acidic, basic, or neutral.