Buffer solution is prepared by mixing

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 $..........$.

What will be the change in $pH$ by adding $0.1 \ M \ NH_4Cl$ to $0.1 \ M \ NH_4OH$ (weak base) solution? (Given: $K_b$ of $NH_4OH = 1.77 \times 10^{-5}$)

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?

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 $pH$? $(K_a(CH_3COOH) = 2 \times 10^{-5})$

The main buffer present in our blood is.....