Which of the following is a buffer solution of strong acidic nature?

$3 \; g$ of acetic acid is added to $250 \; mL$ of $0.1 \; M \; HCl$ and the solution is made up to $500 \; mL$. To $20 \; mL$ of this solution,$\frac{1}{2} \; mL$ of $5 \; M \; NaOH$ is added. The $pH$ of the solution is: [Given: $pK_{a}$ of acetic acid $= 4.75$,molar mass of acetic acid $= 60 \; g/mol$,$\log 3 = 0.4771$]. Neglect any changes in volume.

The approximate $pH$ of a solution formed by mixing equal volumes of $0.1 \ M$ sodium propanoate and $0.1 \ M$ propanoic acid (if the dissociation constant of propanoic acid is $1.3 \times 10^{-5}$) will be:

$20 \ mL$ solution contains $0.1 \ M \ NH_4Cl$ and $0.01 \ M \ NH_4OH$. By adding which one will its $pH$ not change?

What is a buffer solution? Explain the types of buffer solutions with examples and state their biological importance.

$A$ buffer solution is prepared by mixing $10 \ mL$ of $1.0 \ M$ acetic acid and $20 \ mL$ of $0.5 \ M$ sodium acetate and then diluted to $100 \ mL$ with distilled water. The $pH$ of the buffer solution is ($pK_a$ of acetic acid is $4.76$).