Which of the following does not act as a buffer solution?

Calculate the $pH$ of a buffer solution containing $0.027 \ M$ weak acid and $0.054 \ M$ of its salt with a strong base,if the $pK_{a}$ is $4.2$.

The $pH$ of a buffer solution decreases by $0.02$ units when $0.12 \ g$ of acetic acid is added to $250 \ mL$ of a buffer solution of acetic acid and potassium acetate at $27^{\circ} C$. The buffer capacity of the solution is

$20 \ mL$ of $0.1 \ M$ acetic acid is mixed with $50 \ mL$ of potassium acetate. $K_a$ of acetic acid $= 1.8 \times 10^{-5}$ at $27^{\circ} C$. Calculate the concentration of potassium acetate if the $pH$ of the mixture is $4.8$. (in $M$)

Which of the following may be added to one litre of water to act as a buffer?

Explain the preparation of an acidic buffer solution with an example.