Question

A sample of 7.60 L of NH3 (ammonia) gas at 22 ∘C and 735 torr is bubbled into a 0.450 L solution of 0.400 M HCl (hydrochloric acid).

The Kb value for NH3 is 1.8×10−5.

Assuming all the NH3 dissolves and that the volume of the solution remains at 0.450 L , calculate the pH of the resulting solution. Express your answer numerically to two decimal places.

Answer 1

poH of basic buffer = pkb+ log(NH4+/NH3)

pkb of NH3 = -log(1.8*10^-5) = 4.75

   no of mol of HCl taken = 0.45*0.4 = 0.18 mol

    no of mol of NH4Cl = 0.18 mol

   no of mol of NH3 taken = PV/RT

                          = ((735/760)*7.6)/(0.0821*295)

                          = 0.303 mol

no of mol of NH3 unreacted = 0.303-0.18 = 0.123 mol

                   
pOH = 4.75 + log(0.18/0.123)

      = 4.915

pH = 14-4.915 = 9.085

answer: 9.08