Which of the following solutions does not act as a buffer $:-$

$100 \ mL$ of a solution contains $0.1 \ M$ $NH_4OH$ and $0.1 \ M$ $NH_4Cl$. The $pH$ of the solution will not change on adding :-

$0.1 \ M$ $CH_3COOH$ is titrated against $0.1 \ M$ $NaOH$. What would be the difference in $pH$ between $1/4$ and $3/4$ stages of neutralization of the acid?

If $20 \ mL$ of $0.1 \ M \ NaOH$ is added to $30 \ mL$ of $0.2 \ M \ CH_3COOH$ $(pK_a = 4.74)$,find the $pH$ of the resulting solution.

The buffering action of a mixture of $CH_3COOH$ and $CH_3COONa$ is maximum when the ratio of salt to acid is equal to .......?

Calculate the amount of $(NH_4)_2SO_4$ in grams which must be added to $500 \ mL$ of $0.200 \ M \ NH_3$ to yield a solution with $pH = 9.35$ ($K_b$ for $NH_3 = 1.78 \times 10^{-5}$).