If the pH of a mixture of 10 ml of 0.1 M NH4OH and 10 ml of NH4Cl solution is 8, the pKb value of NH4OH is then closest to

a)3 b)5 c)7 d)9

in aqueous solution

NH₄OH   ---->  NH_{4⁺ + OH^-}

NH_{4Cl  --------->  N}H₄⁺ + cl^_-

 K_{b for NH4OH = [NH4⁺]}[OH^-] /NH₄OH

here common ion effect is observed ....

 K_{b = [NH4Cl][OH^-]}/NH_4OH

     =[OH^-]                                     (NH_{4Cl =NH4OH)}

H⁺ = 10^-8                   (given)

OH^- =K_{w/H⁺ =10^-6}

K_{b = 10^-6}

Pkb =6