Calculate volumes of gaseous reactants

Quantitative chemistry • Volumes of gases

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How to convert a gas volume to moles using molar volume?

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Divide the gas volume in dm3 by 24 dm3 per mole to obtain the number of moles.

Key concepts

What you'll likely be quizzed about

Equal volumes represent equal moles under identical conditions

Gases measured at the same temperature and pressure contain equal numbers of particles in equal volumes, so volume ratios equal mole ratios. Because coefficients in a balanced equation give mole ratios, those coefficients also give volume ratios for gaseous species when conditions match. This relationship removes the need to convert to moles if all relevant substances are gases at the same conditions.

Using coefficients to calculate unknown gas volumes

A balanced chemical equation provides coefficients that show how many moles of each substance react or form. When a volume of one gaseous species is known, the corresponding volume of another gaseous species follows by multiplying or dividing by the ratio of coefficients. For example, if the equation shows 2A(g) + 3B(g) → products and 40.0 dm3 of A(g) is present, the volume of B(g) required equals 40.0 × (3/2) = 60.0 dm3 because the 3:2 coefficient ratio causes the volumes to scale accordingly.

When to use molar volume

Molar volume provides a conversion between moles and volume when only one gas is involved or when standard conditions are needed. At room temperature and pressure, one mole of gas occupies approximately 24 dm3, so volume ÷ 24 dm3 gives moles and moles × 24 dm3 gives volume. Use molar volume when a question requires moles for intermediate steps or when gases are not all treated at the same conditions, but avoid unnecessary conversions when direct use of mole ratios suffices.

Limiting factors and conditions

The equality of volume and mole ratios applies only when temperature and pressure are constant for all gases in the calculation. Changes in temperature or pressure break the direct volume-to-mole correspondence and require use of the ideal gas equation. The rule also fails when a reactant or product is a solid or liquid, because volumes of condensed phases do not follow the same proportional relationship.

Key notes

Important points to keep in mind

Equal volumes of gases at the same temperature and pressure contain equal moles.

Use coefficients from the balanced equation as volume ratios for gaseous species under identical conditions.

Molar volume (≈24 dm3 mol−1 at r.t.p.) converts between moles and volume when needed.

Convert fractional coefficients to whole numbers before applying volume ratios.

Confirm that all species in the ratio are gases and share the same conditions before using direct volume relationships.

If gases are at different conditions, use the ideal gas equation or convert to moles with correct conditions first.

Identify the limiting reactant before calculating actual product volumes if quantities are restricted.

Keep units consistent: convert cm3 or litres to dm3 where necessary before using 24 dm3 mol−1.