Which of the following would produce a buffer solution when mixed in equal volumes?

$A$ buffer solution is prepared by mixing $0.01 \ M$ $HCN$ and $0.02 \ M$ $NaCN$. If $K_{a}$ for $HCN$ is $6.6 \times 10^{-10}$,what is the concentration of $H^{+}$ ions in the solution?

$20$ mL of a solution of acetic acid required $28.4$ mL of $0.1$ $M$ NaOH for its neutralization. $A$ solution $(X)$ was prepared by mixing $20$ mL of the above acetic acid and $14.2$ mL of $0.1$ $M$ NaOH solution. What is the pH of the solution $(X)$? ($pK_a$ value of acetic acid is $4.75$).

$A$ buffer solution is prepared by mixing $0.2 \ M$ sodium acetate and $0.1 \ M$ acetic acid. If $pK_{a}$ for acetic acid is $4.7$,find the $pH$.

The $pK_a$ of a weak acid is $4.8$. What should be the ratio of $\frac{[\text{acid}]}{[\text{salt}]}$,if a buffer of $pH = 5.8$ is required?

What is the $OH^-$ ion concentration of a solution containing $0.05 \, M$ ammonium hydroxide and $0.001 \, M$ ammonium chloride? Given $K_b(NH_4OH) = 1.8 \times 10^{-5}$.