The $pK_a$ of $HCN$ is $9.30$. What is the $pH$ of a solution prepared by mixing $2.5 \ mol$ of $KCN$ and $2.5 \ mol$ of $HCN$ in $500 \ mL$ of water (in $.30$)?

Assertion : In a titration of weak acid and $NaOH$,the $pH$ at half equivalence point is $pK_a$.
Reason : At half equivalence point,it forms an acidic buffer and the buffer capacity is maximum where $[acid] = [salt]$.

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

In a buffer solution containing equal concentration of $B^{-}$ and $HB,$ the $K_b$ for $B^{-}$ is $10^{-10}.$ The $pH$ of the buffer solution is:

In a mixture of acetic acid and sodium acetate,the ratio of the concentration of the salt to the acid is increased ten times. Then the $pH$ of the solution:

$50 \, mL$ of $2 \, N$ acetic acid mixed with $10 \, mL$ of $1 \, N$ sodium acetate solution will have an approximate $pH$ of $(K_a = 10^{-5})$