AQA · Chemistry · 8462
GCSE Chemistry (AQA): Gas Calculations
Topic 3 — Quantitative Chemistry. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Gas Calculations - Volume & Moles | Mass, Moles & Mr (2027/28 exams)"; each card deep-links to the moment in the video it came from. Lesson 3.07 of Cognito's AQA GCSE Chemistry (Triple) course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-t_3.07.
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Which equation links mass, moles and relative formula mass?
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Moles = mass ÷ Mr.
What volume does 3.5 moles of chlorine gas occupy at room temperature and pressure?
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84 dm³ (3.5 × 24).
What is the relative formula mass (Mr) of water, H₂O?
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18 (16 for oxygen + 2 × 1 for the two hydrogens).
What volume does 27 g of water vapour occupy at room temperature and pressure?
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36 dm³ — 27 g ÷ Mr 18 = 1.5 moles, then 1.5 × 24 = 36 dm³.
In a reaction where hydrogen is in excess, which reactant is the limiting reagent?
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The other reactant (e.g. nitrogen) — the amount of product depends entirely on the limiting reagent.
What is the equation linking the volume of a gas to its number of moles at room temperature and pressure?
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Volume (dm³) = moles × 24.
Why does the same molar volume of 24 dm³ apply to chlorine, oxygen and water vapour alike?
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At room temperature and pressure one mole of any gas occupies the same volume, 24 dm³, regardless of which gas it is.
When converting between gas volumes only, what shortcut replaces calculating moles?
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Multiply the given volume directly by the molar ratio — twice the moles of a gas occupies twice the volume.
Under what conditions is the molar volume of 24 dm³ valid?
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Only at room temperature and pressure; gases occupy different volumes if temperature or pressure changes.
Where in a balanced equation do you read off the molar ratio?
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From the big numbers (coefficients) in front of the formulae; a missing number means an unwritten 1.
How do you calculate moles of a gas from its volume in dm³?
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Divide the volume by 24, e.g. 60 dm³ ÷ 24 = 2.5 moles.
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