JEE Main 2023 · 10 April · morning · Single correct · +4 / −1
The number of molecules and moles in 2.8375 litres of O₂ at STP are respectively
- (a) 1.505 × 10²³ and 0.250 mol
- (b) 7.527 × 10²² and 0.250 mol
- (c) 7.527 × 10²³ and 0.125 mol
- (d) 7.527 × 10²² and 0.125 mol
Answer and solution
Answer (d) 7.527 × 10²² and 0.125 mol
Solution
Chemistry ma’am · 3 steps
At STP one mole of gas fills 22.7 L, so divide the volume by 22.7 first, then multiply by Avogadro.
- Molar volume. At STP (1 bar, 273.15 K) one mole of any ideal gas occupies 22.7 L. So the number of moles is the volume divided by 22.7 L mol⁻¹.
n = (2.8375 L)/(22.7 L mol⁻¹) = 0.125 mol
- Count molecules. Multiply the moles by Avogadro's number: 0.125 × 6.022 × 10²³ = 7.527 × 10²² molecules of O₂.
- So. There are 7.527 × 10²² molecules and 0.125 mol of O₂, which is option (d).
The idea to keep. Gas volume at STP goes to moles through 22.7 L, and moles go to particles through 6.022 × 10²³. Always ask whether the question wants molecules or atoms.
Answer from the NTA final answer key. Solution by PrepWiser.