Find the pH of the solution obtained when 0.027 L of 0.063 M benzylamine, C7H7NH2, is titrated to the equivalence point with 0.048 M hydrochloric acid. Kb for benzylamine is 4.7×10-10.

pH = 3.12

Explanation:

C7H7NH2   + HCl ---> C7H7NH3+Cl-

moles of C7H7NH2 = 0.027 x 0.063 = 1.7 x 10^-3  mol

moles of C7H7NH2 = moles of HCl at equivalence point

1.7 x 10^-3  = 0.048 x V

V = 0.035 L

volume of HCl = 0.035 L

so C7H7NH3+Cl- salt formed = 1.7 x 10^-3 moles

salt concentration = moles / total volume

= 1.7 x 10^-3 / (0.027 + 0.035)  = 0.027M

this salt formed from weak base strong acid . so pH <7

pH = 7 - 1/2 [pKb + logC]

pH = 7 -1/2 [9.33 + logC]

pH = 3.12