Gay-Lussac’s Law Explained: Combining Volumes of Gases With Simple Examples

 

When studying chemistry, one of the most fascinating aspects is how gases behave during chemical reactions. Even before scientists fully understood atoms and molecules, they noticed that gases react in simple and predictable volume ratios. This observation led to one of the most important gas laws in chemistry:

Gay-Lussac’s Law of Combining Volumes.

This law helps students, teachers, and science enthusiasts predict how gases combine and what volumes of products will be formed — and the best part is that it only requires simple ratios, not complicated calculations.

 What Is Gay-Lussac’s Law of Combining Volumes?

Gay-Lussac’s Law states that: When gases react together under the same conditions of temperature and pressure, they combine in simple whole-number ratios by volume.

This means that if two gases react, you can predict how much of each gas will be needed — and how much product gas will be formed — simply by looking at the reaction equation.

 Basic Ideas you need to know before Applying Gay Lussac's Law

1. Temperature and pressure must remain constant

The volume ratios only hold true when reacting gases are measured under the same conditions.

2. Volumes combine in whole-number ratios

Examples include 1:1, 2:1, 1:3, 3:2, etc.

3. Product gases also follow simple ratios

As long as the products are gases, you can apply the same rule.

These simple patterns exist because equal volumes of gases contain equal numbers of particles according to Avogadro’s principle. So if gases react in simple mole ratios, the volumes behave the same way.

Common Reactions Explained With Volume Ratios

Here are some everyday chemistry reactions where Gay-Lussac’s Law shines.

1 Formation of Water Vapor

2H_2(g) + O_2(g) --->  2H_2O(g)

Volume ratio:

 Hydrogen : Oxygen : Steam

                      2 : 1 : 2

So, twice as much hydrogen is used as oxygen.

2 Formation of Ammonia

N_2(g) + 3H_2(g) ---->  2NH_3(g)

Volume ratio:

 Nitrogen : Hydrogen : Ammonia

                    1 : 3 : 2

This reaction is often used to manufacture fertilizers.

3  Formation of Carbon Dioxide

2CO(g) + O_2(g) -----> 2CO_2(g)

Volume ratio:

Carbon monoxide : Oxygen : Carbon dioxide

           2 : 1 : 2

This means CO and CO2 are equal in volume.

Step-by-Step Calculation Examples

Below are simple examples that show exactly how to use Gay-Lussac’s Law to solve problems quickly.

Example 1

If 60 cm³ of hydrogen reacts completely with oxygen, what volume of oxygen will be needed?

Equation:

2H_2 + O_2 ---->  2H_2O

Ratio:       2H_2 : 1O_2

Calculation:

oxygen  = 1/2 x 60  = 30cm cube

Required oxygen = 30 cm³

Example 2

What volume of ammonia will form from 80 cm³ of nitrogen (with excess hydrogen)?

Ratio:

[N_2 : NH_3 = 1 : 2]

volume of ammonia  formed = 80/2 x 160cm cube

Ammonia produced = 160 cm³

Example 3

If 150 cm³ of carbon monoxide reacts with oxygen, what volume of carbon dioxide will be formed?

Ratio:  [2CO : 2CO_2]

Product equals reactant.

CO2 formed = 150 cm³

Example 4

Calculate the volume of hydrogen needed to react completely with 200 cm³ of nitrogen.

Ratio:   [1 : 3]

volume of hydrogen = 200 x 3= 600cm cube

Hydrogen required = 600 cm³

 Practice Questions

Try solving these for yourself:

1. 100 cm³ of H2 reacts with Cl2. How much Cl2 is needed?

2. 120 cm³ of N2 reacts with H2. How much NH3 forms?

3. 75 cm³ of CO reacts with oxygen. How much CO2 forms?

4. 30 cm³ of O2 reacts with hydrogen. What volume of H2 is required?

Answers

1. 100 cm³

2. 240 cm³

3. 75 cm³

4. 60 cm³

 Final Thoughts

Gay-Lussac’s Law makes gas calculations incredibly easy. Instead of using moles or formulas, all you need are simple whole-number volume ratios. This law is especially helpful for students preparing for exams in chemistry because it turns complex gas behavior into simple arithmetic.