Calculate the pH of \( 0.5 \mathrm{~L} \) of a \( 0.2 \mathrm{M} \m...

Calculate the pH of \( 0.5 \mathrm{~L} \) of a \( 0.2 \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}-0.2 \mathrm{M} \mathrm{NH}_{3} \) buffer before and after addition of (a) \( 0.05 \mathrm{~mole} \) of \( \mathrm{NaOH} \) and (b) \( 0.05 \) mole of \( \mathrm{HCl} \). Assume that the volume remains constant.
[Given : \( \mathrm{pK}_{\mathrm{b}} \) of \( \left.\mathrm{NH}_{3}=4.74\right] \)