Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion : Boric acid is a weak acid.
Reason R : Boric acid is not able to release H⁺ ion on its own. It receives OH⁻ ion from water and releases H⁺ ion.
In the light of the above statements, choose the most appropriate answer from the options given below.

Solution

Chemistry ma’am · 4 steps

Boric acid gives no proton of its own: boron grabs an OH⁻ from water, and that frees the H⁺.

1. Why boron behaves so. In B(OH)₃, boron has only 6 electrons around it and an empty p orbital. So it acts as a Lewis acid, an electron-pair acceptor, not as a proton donor.
2. How H⁺ appears. Boron accepts an OH⁻ from a water molecule. The H⁺ left behind from that water goes into solution.

   B(OH)₃ + 2H₂O ⇌ [B(OH)₄]⁻ + H₃O⁺

3. Why it is weak. This equilibrium lies far to the left (Kₐ ≈ 5.8 × 10⁻¹⁰), so very little H₃O⁺ forms. Boric acid is a weak, monobasic acid, so Assertion A is true.
4. So. R is true, and it is the very reason A is true: the acid can only make H⁺ indirectly through water, and it does so poorly. The answer is (a).

The idea to keep. When a molecule like B(OH)₃ has an electron-deficient centre, ask whether it donates H⁺ or accepts OH⁻. Boric acid accepts OH⁻, so it is a weak, monobasic Lewis acid.

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