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The Mole Concept and Stoichiometry

Topic 4 of 5

Gas volumes and hydrocarbon combustion

Use volume ratios only when temperature and pressure match.

A-Level 9476 (2026-2027)

At the same temperature and pressure, gas volumes follow mole ratios

Water may condense; do not count a liquid as part of the final gas volume.

For gases behaving ideally, V is proportional to n at fixed temperature and pressure. The balanced coefficients therefore also give gas-volume ratios under those conditions. If conditions differ, use pV = nRT first. Do not insert a memorised molar gas volume when the stated temperature and pressure require a different value.

Complete combustion of a hydrocarbon obeys CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O. The carbon dioxide amount gives x; oxygen consumption then gives y. For an oxygen-containing fuel CxHyOz, the oxygen coefficient becomes x + y/4 - z/2.

Worked example

Infer a hydrocarbon from gas-volume changes

At the same temperature and pressure, 10.0 cm3 of a gaseous hydrocarbon burns completely using 35.0 cm3 O2 and producing 20.0 cm3 CO2. Find the formula.

  1. The CO2/fuel volume ratio is 2, so x = 2.
  2. The O2/fuel ratio is 3.5, so x + y/4 = 3.5.
  3. Substitute x = 2: y/4 = 1.5 and y = 6.
Answer

C2H6; 2C2H6 + 7O2 → 4CO2 + 6H2O.

Check your understandingThe same 10.0 cm3 fuel is burnt with 50.0 cm3 O2. After cooling so water condenses, what dry gas volume remains?Think it through, then reveal the answer
15.0 cm3 unreacted O2 plus 20.0 cm3 CO2 = 35.0 cm3, measured at the original temperature and pressure. Absorbing all the CO2 would leave 15.0 cm3. The condensed water is not part of the dry gas volume.