JEE Main 2022 · 29 June · evening · Single correct · +4 / −1
Aqueous solution of which of the following boron compounds will be strongly basic in nature?
- (a) NaBH₄
- (b) LiBH₄
- (c) B₂H₆
- (d) Na₂B₄O₇
Answer and solution
Answer (d) Na₂B₄O₇
Solution
Chemistry ma’am · 3 steps
Ask what each compound gives when it reacts with water. Only borax releases a strong base, NaOH.
- Borax hydrolyses. Borax is the salt of a strong base (NaOH) and a weak acid (H₃BO₃). In water the tetraborate ion takes up water and gives back both of them.
Na₂B₄O₇ + 7H₂O → 2NaOH + 4H₃BO₃
- Why basic. NaOH ionises fully, but boric acid is a very weak acid (Kₐ ≈ 10⁻⁹). The strong base wins, so the solution is strongly alkaline.
Compound With water Nature of solution NaBH₄, LiBH₄ slow reaction, gives H₂ and a metaborate not strongly basic B₂H₆ B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂ weakly acidic Na₂B₄O₇ gives NaOH and H₃BO₃ strongly basic - So. The aqueous solution of borax, option (d), is strongly basic.
The idea to keep. For a salt in water, look at its parent acid and base. Strong base plus weak acid, like borax from NaOH and boric acid, gives an alkaline solution.
Answer from the NTA final answer key. Solution by PrepWiser.