Which of the following pairs constitutes a buffer?

Which of the following buffer solutions will have the highest acidic character?

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 $.30$)?

Which of the following solutions will act as a buffer solution?

Which one of the following pairs of solutions is not an acidic buffer?

$50 \ mL$ of $2 \ N$ acetic acid mixed with $10 \ mL$ of $1 \ N$ sodium acetate solution will have an approximate $pH$ of: