Question

Calculate the concentration of all species present in a 0.01 M aqueous solution of ammonia. The pKa of the ammonium ion is 9.24.

A.)Calculate [H⁺] (express the result in molar units using three significant figures and scientific notation).

B.)Calculate [OH^-] (express the result in molar units using three significant figures and scientific notation).

C.)Calculate [NH+4]  (express the result in molar units using three significant figures and scientific notation).

D.)Calculate [NH3]  (express the result in molar units using three significant figures and scientific notation).

E.)What is the pH of the solution?

Answer 1

          Pka + Pkb = 14

          Pkb               = 14-Pka

                               = 14-9.24

           PKb            = 4.76

          -logKb        = 4.76

                Kb       = 10^-4.76

                           = 1.74*10^-5

      NH3 + H2O ---------> NH4⁺ + OH^-

I        0.01                            0         0

C    -x                                   +x       +x

E    0.01-x                            +x       +x

    Kb = [NH4+][OH-]/{NH3]

   1.74*10^-5 = x*x/0.01-x

1.74*10^-5 *(0.01-x) = x²

          x = 0.00041

[NH4+] =x = 0.000408M

[OH-]    = x   = 0.000408M

{NH3]   = 0.01-x = 0.01-0.000408 = 0.00959M

[H+]   = Kw/[OH-]

           = 1*10^-14/0.000408

           =2.45*10^-11 M

E. PH = -log[H+]

            = -log2.45*10^-11

           = 10.61