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At the same temperature and pressure, equal volumes of gases contain equal numbers of molecules. Under the same conditions, gas-volume ratios follow mole ratios.

$V_gas = nV_m$

Balance the equation, identify the limiting gas amount when needed, apply the coefficient ratio to gas volumes at the same temperature and pressure, then convert to the requested volume units.

For N₂ + 3H₂ → 2NH₃ at one temperature and pressure, one gas volume of N₂ requires three equal gas volumes of H₂ and forms two of NH₃. This direct volume ratio works because the gases share conditions; otherwise convert through moles.

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$140.0 \mathrm{~cm}^{3}$ of ethyne are reacted with $160.0 \mathrm{~cm}^{3}$ of hydrogen in a container at 420 K and $1.00 \times 10^{5} \mathrm{~Pa}$ . Ethane is the only product.

Determine the maximum possible volume of ethane formed under these conditions of temperature and pressure.

Retrieve the quantitative chain: count entities with mN_A, sum relative masses from formulae, convert mass with n=m/M, derive formula ratios, use n=VC for solutions, and apply gas-volume ratios at the same temperature and pressure.

Before finalising, check the requested entity, formula subscripts, units, dm³ conversion, balanced-equation coefficients, and any limiting reactant.

1.4.6—Avogadro's law

Avogadro's Law and Gas Volumes

The Mole Summary

Using Gas-Volume Ratios