Acetic acid is 5% ionised in its decimolar solution. Calculate the dissociation constant of acetic acid.(ii) Write chemical formula of epsom salt. Give two uses of H2SO4.

Answer: (i) \(K_a \approx 2.5\times10^{-4}\) (using \(K_a = c\alpha^2\); the exact expression gives \(2.63\times10^{-4}\)). (ii) MgSO₄·7H₂O; uses below.

(i) \(c = 0.1\) M (decimolar), \(\alpha = 5\% = 0.05\).

By Ostwald’s dilution law, \(K_a = \dfrac{c\alpha^2}{1 - \alpha}\). Since \(\alpha\) is small, \(1 - \alpha \approx 1\):

(Without the approximation, \(K_a = \dfrac{0.1\times0.0025}{0.95} = 2.63\times10^{-4}\).)

(ii) Epsom salt is magnesium sulfate heptahydrate, MgSO₄·7H₂O.

Uses of H₂SO₄ (any two):

1. Manufacture of fertilisers such as ammonium sulfate and superphosphate of lime.
2. As the electrolyte in lead storage batteries.
3. In petroleum refining, and in the manufacture of pigments, paints, dyes and detergents.
4. As a dehydrating agent and in the preparation of other acids (HCl, HNO₃) in the laboratory.