Mole Concept — Part 1: Gas volume laws: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Gases react in simple volume ratios

Part 1 — when gases react, their volumes (measured under the same conditions) are in small whole-number ratios. Two laws explain this, and they lead us to the mole.

In this lesson you will

State Gay-Lussac's law of combining volumes
Calculate the volumes of gases that react or form, using a balanced equation
State Avogadro's law and link gas volumes to numbers of molecules
Explain why hydrogen, oxygen, nitrogen and chlorine are diatomic

Table of four gas reactions and the simple whole-number ratios of their volumes
Reaction (all gases)	Volume ratio
Hydrogen + chlorine → hydrogen chloride	1 : 1 : 2
Nitrogen + hydrogen → ammonia	1 : 3 : 2
Nitrogen + oxygen → nitrogen monoxide	1 : 1 : 2
Carbon monoxide + oxygen → carbon dioxide	2 : 1 : 2

Concept

Gay-Lussac's law of combining volumes

Gay-Lussac's law: when gases react, they do so in volumes that bear a simple whole-number ratio to one another and to the volumes of the gaseous products, provided all the volumes are measured at the same temperature and pressure.

For ammonia: 1 volume of nitrogen + 3 volumes of hydrogen → 2 volumes of ammonia. The ratio of the volumes is the ratio of the coefficients in the balanced equation.

Check your understanding

Quick check: volume ratios

Read the volume ratio straight from a balanced equation.

Question: Multiple choice

Multiple choice1 mark

For the reaction N₂(g) + 3H₂(g) → 2NH₃(g), with all volumes measured at the same temperature and pressure, the volumes of nitrogen, hydrogen and ammonia are in the ratio:

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