Salt Chemistry Calculator — pH and Classification

When a salt dissolves in water, its ions may react with water (hydrolyze) to shift the pH away from 7. Whether a salt is acidic, basic, or neutral depends on the strength of the parent acid and base.

Classification rules:

• Strong acid + strong base: neutral salt. No hydrolysis. pH = 7 (e.g. NaCl, KNO3).
• Strong acid + weak base: acidic salt. The cation hydrolyzes: BH+ + H2O -> B + H3O+. pH < 7 (e.g. NH4Cl, pH about 5).
• Weak acid + strong base: basic salt. The anion hydrolyzes: A- + H2O -> HA + OH-. pH > 7 (e.g. NaCH3COO, sodium acetate, pH about 9).
• Weak acid + weak base: depends on relative Ka and Kb. If Ka > Kb, slightly acidic; if Kb > Ka, slightly basic.

For a basic salt (weak acid + strong base), pH estimate: Ka of the weak acid is known. Kb of conjugate base = Kw / Ka (where Kw = 10^-14). For dilute solution: [OH-] ≈ sqrt(Kb * C), where C is concentration. pH = 14 + log([OH-]) = 14 + 0.5*(log(Kb) + log(C))

For an acidic salt (strong acid + weak base): Ka of conjugate acid = Kw / Kb (base) [H+] ≈ sqrt(Ka * C) pH = 0.5*(pKa - log C) = -0.5*(log(Ka) + log(C))

This estimation assumes dilute solutions and that Ka or Kb « C (the weak acid/base approximation). For concentrated solutions or when Ka/Kb is close to C, a quadratic solution is more accurate.