1.4.6—Avogadro's law

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.

How it is assessed

Questions apply balanced-equation mole ratios to gas volumes at fixed conditions or convert a reaction amount to gas volume at STP.

Command terms

determine / calculate

What earns marks

Identify the limiting reactant where relevant, preserve the balanced-equation ratio, and use the stated molar volume or gas equation with consistent units.

Watch for

Using a mass ratio instead of the balanced gas-volume ratio, or ignoring the limiting gas before calculating product volume.

Representative question

[Maximum number: 2]

$140.0 \mathrm{~cm}^{3}$ 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.

hydrogen is the limiting reactant $\mathrm{OR} 2 \mathrm{H}_{2}: 1 \mathrm{C}_{2} \mathrm{H}_{6} \vee$ $80.0 \mathrm{~cm}^{3} \text { V }$

Marking guidance:

Award [2] for correct final answer.

Accept answers expressed in $d m^{3}$ .

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

Using Gas-Volume Ratios

Assessment in practice

The Mole Summary

Question 1