Salts of strong acid with weak base are acidic, strong base with weak acid are basic.
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The pH of a salt solution depends on whether its parent acid and base are strong or weak. A strong acid combined with a strong base gives a neutral salt solution with pH 7, while a strong acid with a weak base gives an acidic solution and a weak acid with a strong base gives a basic one. When both parents are weak, the result depends on the relative sizes of Ka and Kb, and the stronger parent determines the direction of the shift. Quantitative calculation uses the hydrolysis constant Kh, equal to Kw divided by Ka for an anion or Kw divided by Kb for a cation. The concentration of hydroxide or hydronium ion then follows from Kh. This approach is used in analysis and when planning buffers.
Key ideas: For an anion A- of concentration C: Kb = Kw/Ka and [OH-] is approx sqrt(Kb C). For a cation BH+: Ka = Kw/Kb and [H+] is approx sqrt(Ka C). Example: 0.10 M sodium acetate has Kb 5.6e-10, [OH-] 7.5e-6, pH 8.88. Salt of weak acid and weak base: pH is about 7 + 0.5(pKa - pKb).
Common mistakes and connections: Do not use the Ka of the parent acid as though it were the anion's Kb. Another error is forgetting to convert pOH to pH. Hydrolysis results explain why the equivalence point of a weak acid titration lies above 7, and guide indicator selection.
pH of 0.10 M salts reflects the parent acid and base: strong-weak pairs give acidic or basic solutions, while NaCl (strong-strong) stays at 7. The interactive lesson shows this as a live animation or chart.
A 0.10 M sodium acetate solution has an OH- concentration of about 7.5 x 10^-6 M and a pH of about 8.9.
Salts of strong acid with weak base are acidic, strong base with weak acid are basic.
pH of Salt Solutions is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Acids, Bases & Electrochemistry first: Polyprotic Acids, Acid Strength and Structure, Hydrolysis of Salts.
The next topics in Acids, Bases & Electrochemistry are Acid-Base Neutralization and Titration.
Yes. The interactive lesson includes a self-check question on pH of Salt Solutions, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
pH of Salt Solutions is topic 20 of 105 in Acids, Bases & Electrochemistry.