At the same temperature and pressure, a mole of hydrogen fills the same space as a mole of carbon dioxide. That single fact turns gas volumes into moles, and lets you read volume ratios straight from an equation.
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. So one mole of any gas occupies the same volume: 24.0 dm3 at room conditions, the Data Booklet value. Amount of gas n = V ÷ 24.0 with V in dm3 (or V ÷ 24 000 with V in cm3). For reactions between gases, the volume ratio equals the mole ratio in the equation.
Set the amount of gas in the piston and read the volume at room temperature and pressure. Double the moles and the volume doubles. Then change the temperature or pressure and see why every molar volume must quote its conditions.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What volume does 0.50 mol of nitrogen occupy at room conditions? (molar volume 24.0 dm3 mol−1)
How many moles are there in 600 cm3 of a gas at room conditions?
Avogadro's law states that, at the same temperature and pressure, equal volumes of gases contain:
In N2(g) + 3H2(g) → 2NH3(g), what volume of nitrogen reacts with 30 cm3 of hydrogen at the same temperature and pressure?
When every volume is measured at the same temperature and pressure, the coefficients in the equation give the volume ratio directly. No conversion to moles is needed.
When one substance is a solid or solution and the other a gas, go through moles as usual and finish with the molar volume.
Watch the units: 1 dm3 = 1000 cm3, so use 24.0 with dm3 and 24 000 with cm3. Water formed in a combustion reaction is a liquid at room temperature, so it does not count in the final gas volume. And the molar volume applies only to gases, never to solids, liquids or solutions.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What volume of oxygen is needed to burn 20 cm3 of propane completely, all volumes at the same temperature and pressure? C3H8 + 5O2 → 3CO2 + 4H2O
1.00 g of calcium carbonate (Mr 100.1) reacts with excess hydrochloric acid. What volume of carbon dioxide forms at room conditions?
At room conditions, 1.20 dm3 of a gas has a mass of 2.20 g. What is its relative molecular mass?
10 cm3 of a gaseous hydrocarbon uses exactly 45 cm3 of oxygen and forms 30 cm3 of carbon dioxide, all at room conditions. What is the hydrocarbon?
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