Question

Sol. Key concept: At very low acid/base concentration, water ionisation cannot be neglected.

At 25°C, take K_w = 10⁻¹⁴ and C = 10⁻⁸ mol L⁻¹. For HCl, charge balance gives [H⁺] = C + [OH⁻], together with [H⁺][OH⁻] = K_w.

$$\lbrack H^{+}\rbrack = \frac{C + \sqrt{C^{2} + 4K_{w}}}{2} = 1.05125 \times 10^{- 7}\ mol\ L^{- 1}$$

Thus pH ≈ 6.978 for 10⁻⁸ M HCl, making (b) true. The corresponding NaOH solution has pOH ≈ 6.978 and pH ≈ 7.022, so (a) is false.

(c) True: hydrated Fe³⁺ hydrolyses water and releases H⁺, making an FeCl₃ solution acidic.

(d) False as an acid–base neutralization statement: H₃PO₂ has only one ionizable O–H hydrogen. Its other hydrogens are P–H hydrogens, so NaH₂PO₂ is not an acid salt that forms Na₂HPO₂ by further neutralization. Hence (b) and (c).