Which of the following statement(s) is/are correct?
(A) The pH of 1 × 10⁻⁸ M HCl solution is 8 .
(B) The conjugate base of H₂ PO₄⁻ is HPO₄²⁻.
(C) K_w increases with increase in temperature.
(D) When a solution of a weak monoprotic acid is titrated against a strong base at half neutralisation point, pH=1/2 pKₐ
Choose the correct answer from the options given below:

Solution

Chemistry ma’am · 2 steps

Test each statement against one idea: water's own ions, proton loss, endothermic ionisation, buffer equation.

1. Very dilute acid. At 10⁻⁸ M, the acid gives fewer H⁺ ions than water itself does (10⁻⁷ M), so water cannot be ignored. Let water supply x; then x(x + 10⁻⁸) = 10⁻¹⁴, which gives x ≈ 0.95 × 10⁻⁷ M.

   [H⁺] ≈ 1.05 × 10⁻⁷ M ⇒ pH ≈ 6.98

   Statement	Check	Verdict
   (A) pH is 8	An acid cannot make water basic. The true pH is about 6.98	Incorrect
   (B) Conjugate base of H₂PO₄⁻ is HPO₄²⁻	Removing one H⁺ from H₂PO₄⁻ gives HPO₄²⁻	Correct
   (C) K_w rises with temperature	H₂O ⇌ H⁺ + OH⁻ is endothermic, so heating pushes it forward	Correct
   (D) pH = 1/2pKₐ at half neutralisation	Here [A⁻] = [HA], so pH = pKₐ + log 1 = pKₐ	Incorrect

2. So. Only (B) and (C) are correct, which is option (b).

The idea to keep. When an acid or base is below 10⁻⁶ M, add water's own H⁺ or OH⁻. And at half neutralisation, pH equals pKₐ exactly, with no factor of one half.

Answer from the NTA final answer key. Solution by PrepWiser.