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 combinations will form a basic buffer solution?

If a small amount of sodium acetate is added to an equimolar mixture of sodium acetate and acetic acid,the $pH$ of the resulting solution will .......

The dissociation constant of acetic acid is $x \times 10^{-5}$. When $25 \ mL$ of $0.2 \ M \ CH_3COONa$ solution is mixed with $25 \ mL$ of $0.02 \ M \ CH_3COOH$ solution,the $pH$ of the resultant solution is found to be equal to $5$. The value of $x$ is $..........$.

Observe the following four solutions :
$P : 10 \ mL, 0.1 \ M \ NaOH + 5 \ mL, 0.1 \ M \ HCl$
$Q : 10 \ mL, 0.1 \ M \ NaOH + 15 \ mL, 0.1 \ M \ CH_3COOH$
$R : 10 \ mL, 0.1 \ M \ NH_3 + 10 \ mL, 0.1 \ M \ NH_4Cl$
$S : 10 \ mL, 0.05 \ M \ NaF + 5 \ mL, 0.1 \ M \ HF$
Which of above solutions act as buffer?

Calculate the $pH$ of a buffer solution prepared by mixing $30 \ mL$ of $0.1 \ M$ $NaOH$ and $100 \ mL$ of $0.1 \ M$ $CH_3COOH$. (Given: $pK_a$ of $CH_3COOH = 4.76$)