JEE Main 2021 · 24 February · 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 : Hydrogen is the most abundant element in the Universe, but it is not the most abundant gas in the troposphere.
Reason R : Hydrogen is the lightest element.
In the light of the above statements, choose the correct answer from the options given below :
- (a) Both A and R are true but R is NOT the correct explanation of A
- (b) Both A and R are true and R is 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 (b) Both A and R are true and R is the correct explanation of A
Solution
Chemistry ma’am · 4 steps
Hydrogen is so light that Earth's gravity cannot hold it, and that is exactly why the air has so little of it.
- Check A. Hydrogen makes up about 70% of the mass of the universe, so it is the most abundant element there. In the troposphere, though, N₂ (about 78%) and O₂ (about 21%) dominate, and free H₂ is only a trace. So A is true.
- Check R. Hydrogen has atomic mass 1, the smallest of all elements, and H₂ (molar mass 2 g mol⁻¹) is the lightest gas. So R is true.
- Link them. At the same temperature, lighter molecules move faster, because average speed goes as √(T/M). Being the lightest, many H₂ molecules reach speeds high enough to escape Earth's gravity, so free hydrogen leaks into space. On Earth most hydrogen stays only as compounds such as water.
- So. Both statements are true, and the lightness in R is the reason hydrogen is scarce in the troposphere. The answer is (b).
The idea to keep. In assertion-reason questions, judge each statement on its own first, then ask whether R answers the 'why' of A. Here, light gas means fast molecules, and fast molecules escape gravity.
Answer from the NTA final answer key. Solution by PrepWiser.