Which of the following mixtures in water acts as a basic buffer?

The $pH$ of a buffer solution made by mixing $25 \ mL$ of $0.02 \ M$ $NH_4OH$ and $25 \ mL$ of $0.2 \ M$ $NH_4Cl$ at $25^{\circ}C$ is ($pK_b$ of $NH_4OH = 4.8$).

The $pH$ of a buffer solution prepared by mixing $50 \ mL$ of $0.2 \ M$ $CH_3COOH$ and $250 \ mL$ of $CH_3COONa$ is $4.8$. What is the concentration of $CH_3COONa$ (in $M$)? Given that the $pKa$ of $CH_3COOH$ is $4.8$.

To obtain a buffer which should be suitable for maintaining a $pH$ of about $4 - 5$,we need to have in solution,a mixture of

Calculate the $pH$ of a buffer solution containing $0.01 \ M$ salt and $0.004 \ M$ weak acid. $(pK_{a} = 4.762)$

Which of the following is not a buffer solution?