A 0.10 M solution of ammonium chloride (salt of a weak base and a strong acid) has what approximate pH?

A 0.10 M solution of ammonium chloride (salt of a weak base and a strong acid) has what approximate pH?

• A. 7.00
• B. 4.00
• C. 5.13
• D. 6.50

Answer: (C)

In a salt from a weak base and a strong acid, the anion is a spectator while the cation behaves as a weak acid in water. Here, ammonium chloride releases NH4+, which can donate a proton to water: NH4+ + H2O ⇌ NH3 + H3O+. The acidity of the solution comes from this NH4+ hydrolysis. The strength of NH4+ as an acid is given by Ka(NH4+) = Kw / Kb(NH3). With Kb for ammonia about 1.8×10^-5 and Kw = 1.0×10^-14, Ka ≈ 5.6×10^-10. For a 0.10 M solution of a weak acid, the hydronium concentration is approximated by [H+] ≈ sqrt(Ka × C) = sqrt(5.6×10^-10 × 0.10) ≈ 7.5×10^-6 M. The pH is then −log10(7.5×10^-6) ≈ 5.13. So the solution is acidic, with a pH around 5.13, which aligns with the given answer.

In a salt from a weak base and a strong acid, the anion is a spectator while the cation behaves as a weak acid in water. Here, ammonium chloride releases NH4+, which can donate a proton to water: NH4+ + H2O ⇌ NH3 + H3O+. The acidity of the solution comes from this NH4+ hydrolysis.

The strength of NH4+ as an acid is given by Ka(NH4+) = Kw / Kb(NH3). With Kb for ammonia about 1.8×10^-5 and Kw = 1.0×10^-14, Ka ≈ 5.6×10^-10. For a 0.10 M solution of a weak acid, the hydronium concentration is approximated by [H+] ≈ sqrt(Ka × C) = sqrt(5.6×10^-10 × 0.10) ≈ 7.5×10^-6 M. The pH is then −log10(7.5×10^-6) ≈ 5.13.

So the solution is acidic, with a pH around 5.13, which aligns with the given answer.