Question

C. Calculate the pH of a 0.10 M solution of hydrazine, N2H4. Kb for hydrazine is 1.3

Answer 1

N2H4 + H2O ------------------------> N2H5+ + OH-

0.1                                                 0               0 -----------------------> initial

-x                                                   +x             +x

0.1-x                                             x               x -------------------------> equilibrium

Kb = [N2H5+][OH-]/[N2H4]

Kb = (x)(x)/(0.1-x)

1.3 x 10^-6 = x^2/(0.1-x)

x^2 +1.3 x 10^-6 x   -1.3 x 10^-7 = 0

x= 3.59 x 10^-4

[OH-]= x = 3.59 x 10^-4 M

pOH = -log[OH-]

        = -log(3.59 x 10^-4 )

       = 3.44

pH + pOH = 14

pH = 14-pOH

      = 14-3.44

      =10.55

pH = 10.55