Buffer solution is prepared by mixing

$A$ solution of $H_3BO_3$ and borax is known as:

$A$ buffer solution contains equal concentrations of weak acid and its salt with a strong base. Calculate the $pH$ of the buffer solution if the dissociation constant of the weak acid is $1.8 \times 10^{-5}$.

What is $[H^{+}]$ of a solution that is $0.01 \ M$ in $HCN$ and $0.02 \ M$ in $NaCN$ ($K_a$ for $HCN = 6.2 \times 10^{-10}$)?

In a mixture of a weak acid and its salt,the ratio of concentration of acid to salt is increased ten-fold. The $pH$ of the solution

$20 \, mL$ of $0.2 \, M \, HCN$ is mixed with $10 \, mL$ of $0.2 \, M \, NaOH$. Calculate the $pH$ of the resulting mixture. The $pKa$ value of $HCN$ is $5$.