JEE Main 2022 · 25 June · evening · Single correct · +4 / −1
Given below are two statements one is labelled as Assertion A and the other is labelled as Reason R :
Assertion A : The amphoteric nature of water is explained by using Lewis acid/base concept.
Reason R : Water acts as an acid with NH₃ and as a base with H₂S.
In the light of the above statements choose the correct answer from the options given below :
- (a) Both A and R are true and R is the correct explanation of A.
- (b) Both A and R are true but R is NOT the correct explanation of A.
- (c) A is true but R is false.
- (d) A is false but R is true.
Answer and solution
Answer (d) A is false but R is true.
Solution
Chemistry ma’am · 3 steps
Watch what moves between water and its partner: it is a proton, so this is Brønsted–Lowry, not Lewis.
- Check Reason R. With NH₃, water gives away a proton, so it acts as an acid. With H₂S, water takes a proton, so it acts as a base. R is true.
H₂O + NH₃ ⇌ OH⁻ + NH₄⁺ (water donates H⁺) ; H₂O + H₂S ⇌ H₃O⁺ + HS⁻ (water accepts H⁺)
- Check Assertion A. Both reactions are proton transfers. Giving and taking a proton is the Brønsted–Lowry idea of acids and bases. The Lewis concept talks about accepting and donating electron pairs, and that is not how water's amphoteric nature is explained. A is false.
- So. A is false and R is true, which is option (d).
The idea to keep. Amphoteric water means it can donate a proton or accept one, depending on its partner. Proton transfer belongs to the Brønsted–Lowry concept, not the Lewis concept.
Answer from the NTA final answer key. Solution by PrepWiser.