20 mL of 0.1 M NH4OH is mixed with 40 mL of 0.05 M HCl. The pH of the mixture is nearest to:

(Given: K_b(NH₄OH) = 1 × 10^–5 , log 2 = 0.30, log 3 = 0.48, log 5 = 0.69, log 7 = 0.84, log 11 =1.04)

                             ${\mathrm{NH}}_4\mathrm{OH}+\mathrm{HCl}\to {\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$

initial mmole           2              2

Final                            -               -             2 mmole

$\left[{\mathrm{NH}}_4^{+}\right]=\frac{2\mathrm{mmole}}{60\mathrm{ml}}=\frac{1}{30}\mathrm{M}$

$\mathrm{pH}=\frac{{\mathrm{pK}}_{\mathrm{w}}-{\mathrm{pK}}_{\mathrm{b}}-\log \mathrm{C}}{2}=\frac{14-5+1.48}{2}=5.24$