pH of a Salt Solution Calculator
pH of a salt from a weak acid or weak base by hydrolysis — Kb of the conjugate base or Ka of the conjugate acid from the parent constant, degree of hydrolysis, and the weak-acid/weak-base case.
How it works
weak acid salt: [OH⁻] = √(Kw·c/Ka) → pH = 7 + ½(pKa + log c); weak base salt: [H⁺] = √(Kw·c/Kb) → pH = 7 − ½(pKb + log c); both weak: pH = 7 + ½(pKa − pKb)
The conjugate of a weak acid is a weak base, so acetate pulls protons from water and sodium acetate solutions are mildly alkaline; ammonium is a weak acid, so ammonium chloride is acidic. The hydrolysis constant is Kw divided by the parent’s constant, and the pH follows as for any weak acid or base. For salts where both ions hydrolyse the concentration cancels and the pH depends only on which parent is weaker. Salts of strong acids and strong bases do nothing to water.
Worked example
0.1 M sodium acetate (Ka = 1.8 × 10⁻⁵): Kb = 5.6 × 10⁻¹⁰, [OH⁻] = 7.5 × 10⁻⁶ M, pH = 8.87. 0.1 M NH₄Cl gives pH 5.13 by the mirror-image calculation.
Frequently asked questions
Why is ammonium acetate almost neutral?
Acetic acid and ammonia have the same Ka and Kb (1.8 × 10⁻⁵), so pH = 7 + ½(4.74 − 4.74) = 7.00.
Does dilution change the pH of these salts?
Yes, toward 7 for the one-weak-parent cases (log c term); not at all for the both-weak case.
Related calculators
For study and planning — check units and conditions against your data source before using a result in the lab.