The range of most suitable indicator which should be used for titration of NaX (0.1 M, 10 ml) with 0 — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

This is a titration of a weak base (NaX) with a strong acid (HCl). $10\text{ ml}$ of $0.1\text{ M}$ NaX requires $10\text{ ml}$ of $0.1\text{ M}$ HCl, yielding a total volume of $20\text{ ml}$. The resulting weak acid HX has a concentration $C = 0.05\text{ M}$. $K_a(\text{HX}) = \frac{K_w}{K_b} = \frac{10^{-14}}{10^{-6}} = 10^{-8}$. At the equivalence point, $[\text{H}^+] = \sqrt{K_a C} = \sqrt{10^{-8} \times 0.05} = \sqrt{5 \times 10^{-10}} \approx 2.23 \times 10^{-5}\text{ M}$. $\text{pH} = -\log(2.23 \times 10^{-5}) \approx 4.65$. The most suitable indicator must cover this pH in its transition range, making 3–5 the best option.