JAMB 2020 Chemistry · Question 36
The volume occupied by 1.58g of a gas at S.T.P is 500cm. What is the relative molecular mass of the gas? [ G.M.V at S.T.P = 22.4dm ]
Options
- A.28
- B.32
- C.44
- D.71
Answer: D. 71
Explanation written with AI and checked against the answer key
Analyze the question: We're given the mass of a gas (1.58 g) and its volume at S.T.P. (500 cm³). We need the relative molecular mass, using the molar gas volume of 22.4 dm³ at S.T.P. Concept: At S.T.P., 1 mole of any gas occupies 22.4 dm³ (22,400 cm³). The molar mass is the mass of that one mole of gas. Solution: Step 1: Convert the volume to the same unit as the molar volume. 22.4 dm³ = 22,400 cm³ Step 2: Set up a proportion. 500 cm³ of the gas has mass 1.58 g 22,400 cm³ of the gas has mass x g x = (1.58 × 22,400) / 500 x = 35,392 / 500 x = 70.78 ≈ 71 g/mol So the relative molecular mass is about 71, which matches Cl₂ (2 × 35.5 = 71). Cross-check using moles: Moles = 0.500 dm³ / 22.4 dm³ = 0.02232 mol Molar mass = 1.58 / 0.02232 = 70.8 ≈ 71 ✔ Answer: The relative molecular mass is 71, which is option D. Tip: In gas-volume questions, always check your units first. Convert dm³ to cm³ (×1000) or the other way round before dividing. Also, when your answer lands near a familiar value like 71, think of common gases: Cl₂ = 71, O₂ = 32, CO₂ = 44, N₂ = 28. That helps you confirm the result quickly.AI can make mistakes. Report anything that looks wrong.
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