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Avogadro and the Ideal Gas Law

Woman blowing up a balloon

Woman blowing up a balloon (Deagreez, iStockphoto)

Learn about the theory of Avogadro’s Law and the Ideal Gas Law and explore examples in everyday life.

There are four laws, known as Gas Laws, which describe how gases behave. The four laws are Boyle’s Law, Charles’s Law, Gay-Lussac’s Law and Avogadro’s Law.

Avogadro’s Law 

Amadeo Avogadro was an Italian physicist who stated, in 1811, that the volume of any gas is proportional to the number of molecules of gas (measured in Moles – symbol mol). In other words if the amount of gas increases, then so does its volume. 

Avogadro’s Law in action
Avogadro’s Law in action (©2020 Let’s Talk Science).

One important lesson we learn from this law is that if samples of any gas are compared that have the same volume, temperature and pressure, then they will all have the same number of molecules in them. You can see this on the following chart:

Chart comparing the masses of argon gas, oxygen gas and nitrogen gas at the same volume, quantity, pressure and temperature
Chart comparing the masses of argon gas, oxygen gas and nitrogen gas at the same volume, quantity, pressure and temperature (©2020 Let’s Talk Science).

Here are three different gases all occupying the same volume (22.4 L), at the same pressure (1 atm) and the same temperature (273 K or 0 °C). Even though their masses are all different, the amounts of each gas are the same (1 mol). One mole (SI unit for the amount of a substance) of a gas (or any substance for that matter), contains the same number of molecules. So we can say that these three gases all contain the same number of molecules. The number of molecules in 1 mole is known as Avogadro’s Number, and is huge (6.02x1023)! Note, we are not restricted to these conditions. For example, we could change the volume for these gases to be higher or lower and change the pressure and temperature to be equally higher or lower as well. As long as all three conditions are the same for all gases, we can then say that they all have the same number of molecules! The number may not be Avogadro’s number any more, but the number of molecules can still be calculated.

Avogadro’s Law in Everyday Life

You have probably experienced this example of Avogadro’s Law yourself. When you blow up a balloon, you are adding molecules of gas into it. The result is that the volume of the balloon increases – and in order to do this, you decrease the number of molecules in your lungs (which decreases their volume)! A bicycle pump does the same thing to a bicycle tire. 

Girl blowing up a balloon/Une petite fille qui gonfle un ballon
Girl blowing up a balloon (Source: Renato Ganoza [CC BY] via Wikimedia Commons).

Ideal Gas Law

The volume, pressure, temperature, and quantity (amount) of gas all can affect one another. Since different gases act similarly, it is possible to write a single equation relating all of these properties. The Ideal Gas Law combines several laws, including Boyle's LawCharles’ Law, Gay-Lussac’s Law and Avogadro’s Law, into one neat and tidy formula! This law is commonly used to calculate how the volume of a gas will change if temperature, pressure or amount of gas is changed.

In the Ideal Gas Law, the behaviour of a gas can be summarized using the following equation:

PV = nRT


  • P is pressure
  • V is volume
  • n is the number of gas molecules in moles
  • R is a number known as the ideal gas constant (The value for R is often, but not always, 8.314 J/mol ˖ K)
  • T is temperature (which has to be in Kelvin)

The chart below shows how all the above gas laws are present in this ideal gas law.



Symbols in Formula


pressure & volume

P, V


volume & temperature

V, T


pressure & temperature

P, T


volume & amount

V, n

Although in reality no gas is an ‘ideal gas’, some do come very close. Therefore, the Ideal Gas Law allows us to roughly predict the behaviour of a gas.

This formula is often used when you want to determine the amount of gas that is present in a container. For example, you could find the mass of gas in a container by weighing the gas in the container, pumping out the gas and then reweighing the container. However, since gases have such low weight, the difference would be so small it would be hard to measure. Instead, all you need to know is the pressure - which can be obtained from a pressure gauge - the volume of the container, and the temperature of the gas. Then put these values into the formula and solve for n, from which the mass can be obtained. 


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