JEE Main · 2020 · 03 Sep 2020 · Shift 1 · Equilibrium

An acidic buffer needs a weak acid together with its conjugate base. Mixing $100$ mL of $0.1$ M $\mathrm{HCl}$ ($0.01$ mol) with $200$ mL of $0.1$ M $\mathrm{C}{\mathrm{H}}_3\mathrm{COONa}$ ($0.02$ mol) lets the strong acid convert part of the acetate, $\mathrm{HCl}+\mathrm{C}{\mathrm{H}}_3\mathrm{COONa}\to \mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{NaCl}$. This leaves $0.01$ mol of acetic acid beside the remaining $0.01$ mol of acetate, a weak acid with its conjugate base, which is exactly an acidic buffer.