Which of the following does not act as a buffer solution?

The $pK_b$ of $CN^{\Theta}$ is $4.7$. The $pH$ of a solution prepared by mixing $2.5 \ mol$ of $KCN$ and $2.5 \ mol$ of $HCN$ in water and making the total volume up to $500 \ mL$ is:

$A$ certain buffer solution contains equal concentrations of $X^-$ and $HX$. If the $K_a$ of $HX$ is $10^{-8}$,then the $pH$ of the buffer solution will be ...... .

$A$ solution of formic acid is half-neutralized by the addition of a $KOH$ solution. If $K_a (HCOOH) = 2 \times 10^{-4}$,what is the $pH$ of the solution? (Given: $\log 2 = 0.3010$)

Calculate the $pH$ of a buffer solution containing $6.1 \times 10^{-2} \ M$ sodium acetate $(CH_3COONa)$ in $1 \ L$ of $0.1 \ M$ acetic acid $(CH_3COOH)$ solution. (Given: $pK_a$ of $CH_3COOH = 4.76$)

$A$ 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