Which of the following cannot act as a buffer solution?

Find the $pH$ when $0.2 \, mol$ of $HCl$ is added to $1 \, L$ of a solution containing $1 \, M$ $CH_3COOH$ and $1 \, M$ $CH_3COO^-$. Assume the total volume remains $1 \, L$. Given $K_a$ for $CH_3COOH = 1.8 \times 10^{-5}$.

Derive the Henderson-Hasselbalch equation.

If $12.5 \ mL$ of $0.1 \ M$ $KOH$ is added into $50 \ mL$ of $0.1 \ M$ $CH_3COOH$ solution,then what would be the final $pH$? $(K_a(CH_3COOH) = 2 \times 10^{-5})$

How many moles of sodium acetate should be added to $1 \ L$ of a $0.1 \ M$ solution of $CH_3COOH$ to give a solution of $pH = 5.5$? (Given: $pK_a$ of $CH_3COOH = 4.5$)

$A$ buffer solution is prepared in which the concentration of $NH_3$ is $0.30 \ M$ and the concentration of $NH_4^+$ is $0.20 \ M.$ If the equilibrium constant,$K_b$ for $NH_3$ equals $1.8 \times 10^{-5},$ what is the $pH$ of this solution? $(\log \ 2.7 = 0.43)$