Buffer solution is prepared by mixing

At $25 \, ^\circ C$,the $K_a$ for $HCN$ is $5 \times 10^{-10}$. What volume (in $mL$) of $5 \, M \, KCN$ solution must be added to $10 \, mL$ of $2 \, M \, HCN$ solution to maintain a constant $pH$ of $9$?

$A$ solution of $0.1 \ mol$ of $CH_3NH_2$ $(K_b = 5 \times 10^{-4})$ and $0.08 \ mol$ of $HCl$ is diluted to $1 \ L$. The $pOH$ of the resulting solution is $(\log 1.25 = 0.1)$.

What is the ratio of $pH$ for a solution containing $1 \ mol$ $CH_3COONa$ + $0.5 \ mol$ $HCl$ per liter and another solution containing $1 \ mol$ $CH_3COONa$ + $1 \ mol$ $CH_3COOH$ per liter?

Amongst the following solutions,the buffer solution is

$A$ buffer solution contains $0.1 \ mol$ of sodium acetate in $1000 \ cm^{3}$ of $0.1 \ M$ acetic acid. To the above buffer solution,$0.1 \ mol$ of sodium acetate is further added and dissolved. The $pH$ of the resulting buffer is equal to