JEE Main 2021 · 24 February · morning · Numerical answer · +4 / −0
Number of amphoteric compounds among the following is ________.
(A) BeO
(B) BaO
(C) Be(OH)₂
(D) Sr(OH)₂
Answer and solution
Answer 2
Solution
Chemistry ma’am · 3 steps
Only beryllium breaks the group trend, so only its oxide and hydroxide are amphoteric.
- What amphoteric means. An amphoteric compound reacts with both acids and bases. In group 2, the oxides and hydroxides become more basic as you go down the group.
- Why beryllium differs. Be²⁺ is very small and highly polarising, so its bonds have strong covalent character. That is why BeO and Be(OH)₂ dissolve in acids and in alkalis alike.
Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O Be(OH)₂ + 2NaOH → Na₂[Be(OH)₄]
Compound Metal Nature BeO Be amphoteric BaO Ba basic Be(OH)₂ Be amphoteric Sr(OH)₂ Sr basic - So. Only BeO and Be(OH)₂ are amphoteric, so the count is 2.
The common slip: 1. Counting only the oxide BeO and missing that Be(OH)₂ is also amphoteric gives 1, not 2.
The idea to keep. In group 2, beryllium is the odd one out. Its oxide and hydroxide are amphoteric, while those of Mg to Ba are basic, and basic strength rises down the group.
Answer from the NTA final answer key. Solution by PrepWiser.