A 25.0 mL sample of 0.400 M NH₃(aq)is titrated with 0.400 M HCl(aq).What is the pH at the equivalence point? (K_b of NH₃ = 1.8 × 10^-5)

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Quizplus · Accepted Answer

At the equivalence point in a titration between a weak base (NH3) and a strong acid (HCl), the solution contains the conjugate acid of the weak base (NH4+). The pH is determined by the hydrolysis of NH4+. Using the given Kb for NH3 (1.8 × 10^-5), we can find the Ka for NH4+ using the relation Kw = Ka * Kb, where Kw = 1.0 × 10^-14 at 25°C. Ka for NH4+ = 1.0 × 10^-14 / 1.8 × 10^-5 = 5.56 × 10^-10. The pH is calculated using the formula pH = 1/2 (pKa - log [salt]), where [salt] is the concentration of NH4Cl at the equivalence point. Since the initial concentrations of NH3 and HCl are equal and the volume of NH3 solution is not changed significantly by the addition of HCl, the concentration of NH4Cl at the equivalence point is approximately the same as the initial concentration of NH3, which is 0.400 M. Therefore, pH = 1/2 (9.25 - log (0.400)) ≈ 4.97.

A 250 ML Sample of 0