Which of the following mixtures forms an acidic buffer?

When $0.02 \ mol$ of $NaOH$ is added to $1 \ L$ of a buffer solution,its $pH$ changes from $5.75$ to $5.80$. What is the buffer capacity of the solution?

Assertion $(A)$: The $pH$ of a buffer solution containing equal moles of acetic acid and sodium acetate is $4.8$ ($pK_a$ of acetic acid is $4.8$).
Reason $(R)$: The ionic product of water at $25^{\circ} C$ is $10^{-14} \ mol^2 \ L^{-2}$. The correct answer is

The $pK_a$ of a weak acid $(HA)$ is $4.5$. The $pOH$ of an aqueous buffer solution of $HA$ in which $50\%$ of the acid is ionized is

$A$ buffer solution is formed by a weak acid and its salt with a strong base,or a weak base and its salt with a strong acid. In which of the following pairs is this characteristic found?

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 $.30$)?