$$\eqalign{ & A{g^ + } + N{H_3} \rightleftharpoons {\left[ {Ag\left( {N{H_3}} \right)} \right]^ + }\,;\,{k_1} = 6.8 \times {10^{ - 3}} \cr & {\left[ {Ag\left( {N{H_3}} \right)} \right]^ + } + N{H_3} \rightleftharpoons {\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]^ + };{k_2} = 1.6 \times {10^{ - 3}} \cr} $$
then the formation constant of $${\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]^ + }\,is$$

Solution :
$$\eqalign{ & {\text{The required reaction is}} \cr & A{g^ + } + 2N{H_3} \rightleftharpoons {\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]^ + };\,K = ? \cr & {\text{From the given equations, we have}} \cr & {k_1} = \frac{{{{\left[ {Ag\left( {N{H_3}} \right)} \right]}^ + }}}{{\left[ {A{g^ + }} \right]\left[ {N{H_3}} \right]}};\, \cr & {k_2} = \frac{{{{\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]}^ + }}}{{{{\left[ {Ag\left( {N{H_3}} \right)} \right]}^ + }\left[ {N{H_3}} \right]}} \cr & \therefore \,{\text{The value of}}\,K\,{\text{is given by}} \cr & K = {k_1} \times {k_2} \cr & = 6.8 \times {10^{ - 3}} \times 1.6 \times {10^{ - 3}} \cr & = 1.08 \times {10^{ - 5}}. \cr} $$