Which of the following is not a buffer solution?

The acidic and basic strength of buffer solutions remains constant because ...

What is the ratio of $pH$ for a solution containing $1 \ mol$ $CH_3COONa$ + $0.5 \ mol$ $HCl$ per liter and another solution containing $1 \ mol$ $CH_3COONa$ + $1 \ mol$ $CH_3COOH$ per liter?

For a buffer solution containing equal concentrations of $B^-$ and $HB$,where the $K_b$ of $B$ is $10^{-10}$,what is the $pH$ of the buffer solution?

$20 \ mL$ of $0.1 \ M$ $NaOH$ is added to $50 \ mL$ of $0.1 \ M$ acetic acid solution. The $pH$ of the resulting solution is $....... \times 10^{-2}$ (Nearest integer).
Given: $pKa$ $(CH_3COOH)$ $= 4.76$,$\log 2 = 0.30$,$\log 3 = 0.48$.

$pH$ of a mixture which is $0.1 \ M$ in $CH_3COOH$ and $0.05 \ M$ in $(CH_3COO)_2Ba$ is [$pK_a$ of $CH_3COOH$ = $4.74$]