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H2 Chemistry mole calculations: limiting reagents and gas quantities

Use the balanced equation as a conversion rule between amounts. When two reactant quantities are supplied, do not assume the smaller number of moles is limiting: compare each amount with its stoichiometric coefficient.

Choose the method

Calculate n/coefficient for each reactant. The smallest value limits the extent of reaction and therefore the maximum product amount.

Theoretical yield assumes the specified reaction goes to completion without competing reactions or losses. Percentage yield = actual/theoretical × 100%.

For gases, use the molar volume supplied or pV = nRT with a consistent unit system. With R = 8.31 J mol⁻¹ K⁻¹, use pressure in Pa, volume in m³ and absolute temperature in K.

Worked examples

Example 1

N₂ + 3H₂ → 2NH₃. A mixture contains 0.20 mol N₂ and 0.45 mol H₂. Calculate the maximum amount of NH₃ if the limiting reagent reacts completely.

Compare 0.20/1 = 0.20 with 0.45/3 = 0.15. Hydrogen is limiting.
Maximum n(NH₃) = 2 × 0.15 = 0.30 mol.
Nitrogen remaining = 0.20 - 0.15 = 0.05 mol. This is a stoichiometric maximum, not a prediction of equilibrium yield.

Example 2

Calculate the volume of 0.100 mol ideal gas at 300 K and 100,000 Pa. Use R = 8.31 J mol⁻¹ K⁻¹.

Rearrange pV = nRT: V = nRT/p.
V = 0.100 × 8.31 × 300/100,000 = 0.002493 m³.
Convert to dm³: 2.493 dm³, or 2.49 dm³ to 3 significant figures.

Try it yourself

A reaction has a theoretical yield of 4.00 g. The isolated dry product has mass 3.20 g. Find the percentage yield.

Show the worked answer

Percentage yield = 3.20/4.00 × 100% = 80.0%.
A lower yield can reflect incomplete conversion, side reactions or losses during isolation; the number alone does not distinguish them.

Common mistakes

Use kelvin in the ideal gas equation.
A limiting-reagent calculation is distinct from an equilibrium calculation. State the completion assumption.

Sources and review

SEAB 2026 syllabus: H2 Chemistry 9476: mole calculations and ideal gases.

Original Rae practice, prepared with AI assistance. Selected numerical results and their displayed working are automatically checked at publication; this does not verify every explanation. Curriculum references checked on 5 September 2026. No teacher review or SEAB endorsement is claimed.

What the syllabus expects

Give definitions for relative atomic, relative isotopic, relative molecular and relative formula mass.
Scope: for an ionic compound, Mr stands for relative formula mass
Define what a mole is by reference to the Avogadro constant.
Work out an element's relative atomic mass once the abundances of its isotopes are supplied.
State what an empirical formula and a molecular formula each mean.
Derive empirical and molecular formulae either from combustion results or from percentage composition by mass.
Put together balanced chemical equations.
Carry out mole-based calculations that involve:
Scope: answers must respect the significant figures set by the question
reacting masses drawn from formulae and equations
volumes of gases, for instance when hydrocarbons burn
the volumes and concentrations of solutions
Use results like those in (g) to infer the stoichiometry of a reaction.

How it's examined

Questions on this topic most often ask you to find, calculate. About 4% of the past-paper style questions in Rae's bank for this subject sit in this topic.

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