Last updated: 2026-07-01

What gas stoichiometry answers

A balanced chemical equation tells you how the amounts of reactants and products relate to one another. When one of those products is a gas, a natural follow-up question is: how much gas — measured as a volume — does the reaction make? Gas stoichiometry answers exactly that, bridging the mole bookkeeping of the equation with the physical volume a gas occupies.

The calculation has two parts. First the mole ratio from the balanced equation converts the amount of a known substance into moles of the gas. Then the ideal gas law turns those moles into a volume at the temperature and pressure where the gas is collected.

The two steps

Step 1 — mole ratio

If the known substance appears with coefficient $a$ and the gas product with coefficient $b$ in the balanced equation, then

The ratio $b/a$ is read directly off the coefficients — nothing else from the equation matters for this step.

Step 2 — ideal gas law

With the moles of gas in hand, the ideal gas law gives the volume:

Here $R$ is the gas constant, $T$ the absolute temperature, and $P$ the pressure. Using $R = 0.082057$ L·atm·mol⁻¹·K⁻¹ keeps the volume in litres when pressure is in atmospheres and temperature in kelvin.

Worked example

Hydrogen peroxide decomposes according to

How many litres of oxygen form when 2 mol of H₂O₂ decomposes at 0 °C and 1 atm?

1. Mole ratio: the known is H₂O₂ with coefficient 2, the gas is O₂ with coefficient 1, so n(O₂) = 2 × 1/2 = 1 mol.
2. Ideal gas law: V = (1 × 0.082057 × 273.15) / 1 = 22.41 L.

So 2 mol of hydrogen peroxide releases about 22.41 L of oxygen at standard temperature and pressure.

Starting from a mass

Reactions are often weighed out rather than measured in moles. To start from a mass, convert it to moles first by dividing by the molar mass, then enter the result as the known amount. For instance, 68 g of H₂O₂ (molar mass 34 g/mol) is 2 mol, the value used in the example above. The mole ratio and gas law then proceed unchanged.

Choosing temperature and pressure

The volume of a gas depends on the conditions it is measured at, so the temperature and pressure entered should match where the gas is collected. Two standard reference points appear constantly in coursework: the older STP of 0 °C and 1 atm gives a molar volume of 22.41 L/mol, while the IUPAC STP of 0 °C and 100 kPa gives 22.71 L/mol. When a problem describes room conditions instead, use those — volume rises in proportion to absolute temperature and falls in inverse proportion to pressure.

When the ideal-gas assumption breaks down

The ideal gas law treats molecules as point particles with no mutual attraction, which holds well at low pressure and high temperature. Near a gas's boiling point or at high pressure, intermolecular forces and finite molecular size cause measurable deviations, and a real-gas model such as the van der Waals equation gives a closer estimate. For the temperatures and pressures of typical classroom and laboratory problems, the ideal-gas result is accurate enough.

Related calculations

The mole-ratio step is the heart of every stoichiometry problem, and the gas-law step is shared with the molar-volume and ideal-gas calculators. When the known amount is given as a mass, a mole–mass conversion sits in front of this calculation; when the gas is one of several products competing for a limiting reactant, a limiting-reagent analysis comes first.