AP Chemistry Salt Hydrolysis

Why some salts make solutions acidic or basic, how to write the reaction, and how to find the pH, in four worked examples.

Dissolving a salt in water does not always give a neutral solution. The pH depends on which ions the salt releases and whether either one reacts with water. The prediction is a one-step test of the parent acid and base, and the numerical pH is an ordinary weak-acid or weak-base equilibrium. The equivalence point of a weak acid titration is the best-known case, covered in titration curve pH calculations; this page generalizes it to any salt.

How Salts Change pH: The Parent Acid and Base Test

Each ion of a salt has a parent: the cation comes from a base and the anion from an acid. An ion only reacts with water if it is the conjugate of a weak parent, because the conjugate of a strong acid or strong base is too weak to matter.

• Strong acid + strong base (NaCl, KNO₃): neither ion reacts, so the solution is neutral.
• Weak acid + strong base (NaF, CH₃COONa): the anion is a weak base and makes OH⁻, so the solution is basic.
• Strong acid + weak base (NH₄Cl): the cation is a weak acid and makes H₃O⁺, so the solution is acidic.
• Weak acid + weak base (NH₄F, NH₄CH₃COO): both ions react, and the larger of K_a (cation) and K_b (anion) decides.

The chart shows calculated pH values for 0.10 M solutions of four salts. The conjugate pair relationship that links the parent to its ion, K_w = K_a × K_b, is the same one in Brønsted-Lowry acids and bases.

Worked Example 1: Classifying Five Salts as Acidic, Basic, or Neutral

Classify solutions of NaCl, NaF, NH₄Cl, KNO₃, and CH₃COONa.

NaCl: HCl and NaOH are both strong, so neutral.
NaF: F⁻ comes from the weak acid HF, so basic.
NH₄Cl: NH₄⁺ comes from the weak base NH₃, so acidic.
KNO₃: HNO₃ and KOH are both strong, so neutral.
CH₃COONa: acetate comes from the weak acetic acid, so basic.

Telling strong from weak parents is the whole skill, and the list is short; it is on the strong acids and bases page. Any acid or base not on that list is weak.

Worked Example 2: Writing the Hydrolysis Equation and Finding the pH of Sodium Fluoride

Write the reaction that makes 0.10 M NaF basic, and find its pH. K_a(HF) = 6.8 × 10⁻⁴.

Reaction: F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq). Na⁺ is a spectator.
K_b of F⁻: K_w/K_a = (1.0 × 10⁻¹⁴) ÷ (6.8 × 10⁻⁴) = 1.5 × 10⁻¹¹.
Equilibrium: K_b = x²/(0.10 − x) ≈ x²/0.10, so x = [OH⁻] = √(1.5 × 10⁻¹¹ × 0.10) = 1.2 × 10⁻⁶ M.
pOH = 5.92, so pH = 8.08.

The assumption holds because x is far smaller than 0.10 (about 0.001% of the fluoride reacts). Setting up the unknowns uses the same ICE table as any weak-base problem; the only new step is getting K_b from the parent acid's K_a.

Worked Example 3: Finding the pH of an Ammonium Chloride Solution

Find the pH of 0.10 M NH₄Cl. K_b(NH₃) = 1.8 × 10⁻⁵.

Reaction: NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq). Cl⁻ is a spectator.
K_a of NH₄⁺: K_w/K_b = (1.0 × 10⁻¹⁴) ÷ (1.8 × 10⁻⁵) = 5.6 × 10⁻¹⁰.
Equilibrium: x = [H₃O⁺] = √(5.6 × 10⁻¹⁰ × 0.10) = 7.5 × 10⁻⁶ M.
pH = −log(7.5 × 10⁻⁶) = 5.13.

The cation acts as a weak acid, so the pH lands below 7. The key switch from Example 2 is using K_w/K_b of the parent base to get the K_a of the cation, then solving for [H₃O⁺] directly rather than for [OH⁻].

Worked Example 4: Deciding for a Salt Whose Ions Are Both Weak

Is a solution of NH₄F acidic, basic, or neutral? What about NH₄CH₃COO? Use K_b(NH₃) = 1.8 × 10⁻⁵, K_a(HF) = 6.8 × 10⁻⁴, and K_a(CH₃COOH) = 1.8 × 10⁻⁵.

Cation: K_a(NH₄⁺) = 5.6 × 10⁻¹⁰ in both salts.
NH₄F: K_b(F⁻) = 1.5 × 10⁻¹¹. Since 5.6 × 10⁻¹⁰ > 1.5 × 10⁻¹¹, the cation wins: acidic.
NH₄CH₃COO: K_b(CH₃COO⁻) = 5.6 × 10⁻¹⁰, equal to the cation's K_a, so the effects cancel: about neutral.

The comparison is between the two ions' constants, not between the parent acid and base constants directly. Ammonium acetate comes out neutral because acetic acid and ammonia happen to have the same strength, K = 1.8 × 10⁻⁵.

Five Salt Hydrolysis Practice Questions With Answers

1. Is a solution of KBr acidic, basic, or neutral?
   Show answer

   Neutral. HBr and KOH are both strong, so neither ion reacts with water.
2. Write the net ionic equation that explains why sodium acetate solution is basic.
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   CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq).
3. Rank 0.10 M NaCl, NH₄Cl, and NaF from lowest to highest pH.
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   NH₄Cl < NaCl < NaF, which is acidic, neutral, basic.
4. K_a of HNO₂ is 4.5 × 10⁻⁴. What is K_b of NO₂⁻?
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   K_b = K_w/K_a = (1.0 × 10⁻¹⁴)/(4.5 × 10⁻⁴) = 2.2 × 10⁻¹¹.
5. Is NH₄NO₃ acidic, basic, or neutral, and which ion is responsible?
   Show answer

   Acidic. NH₄⁺ is the conjugate acid of the weak base NH₃; NO₃⁻ comes from a strong acid and does not react.

Common Salt Hydrolysis Mistakes

• Assuming every salt solution is neutral. Only salts of a strong acid and a strong base are.
• Letting a spectator ion react. Na⁺, K⁺, Cl⁻, and NO₃⁻ do not change the pH.
• Using the parent's K_a where K_b is needed. The ion's constant is K_w divided by the parent's.
• Forgetting to convert pOH to pH after solving for [OH⁻].
• Writing the reaction for the wrong ion, such as hydrolyzing Na⁺, or leaving out water.
• Comparing parent constants for a weak-weak salt. Compare K_a of the cation with K_b of the anion.

Salt questions often appear inside titration and equilibrium problems, so quick classification saves time. To see how a stronger Unit 8 would shift your overall estimate, the AP Chem Score Calculator turns section scores into a 1–5 range.

Related Resources

• AP Chem Score Calculator
• Unit 8 Review: Acids and Bases
• AP Chemistry Strong Acids and Bases
• AP Chemistry Bronsted-Lowry Acids and Bases
• AP Chemistry Titration Curve pH Calculations
• AP Chemistry Buffers
• How to Solve ICE Table Equilibrium Problems
• AP Chemistry Study Guide

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