20. To prepare a buffer of pH \( 8.26 \) amount of \( \left(2 NH _{4}\right)_{2} SO _{4} \) to be added to \( 500 mL \) of \( 0.01 M \left( NH _{4} OH \right) \) solution \( \left[p K_{a}\left( NH _{4}^{+}\right)=9.26\right] \) is
(a) \( 0.05 \) mole
(b) \( 0.025 \) mole
(c) \( 0.10 \) mole
(d) \( 0.005 \) mole
