The $\mathrm{pH}$ of a buffer solution obtained by mixing $50 \mathrm{~mL}$ of $0.1 \mathrm{M} \mathrm{NH}_4 \mathrm{OH}$ and $25 \mathrm{~mL}$ of $2.0 \mathrm{M} \mathrm{NH}_4 \mathrm{Cl}$ is ($\mathrm{pK}_{\mathrm{b}}$ is 4.8)