Avogardro's Law is a principle stated in 1811 by the italian chemist Amedeo Avogadro (1776 - 1856).

"Equal volumes of gases at the same temperature and pressure contain equal number of molecules. regardless of theirr chemical nature and physical properties. This number is 6.022 * 10²³. It is the number of molecules of any gas present in a volume of 22.41 L and is the same for the lightest gas (hydrogen) as for a hearvy gas such as carbon dioxide or bromine. This law only applies to gases."

The law can be summarised mathematically as:

^V/_n = k

Where:

• V is the volume of the gas.
• n is the amount of substance of the gas
• k is a proportionality constant.

The volumes of gases are measured in:

• cubic centimeters (cm³)
• cubic decimeters (dm³)
• liters (L)

1 litre = 1 dm³ = 1000 cm³

Methane (natural gas) burns in air according to the equation:

CH₄ (g) + 2O₂ (g)  --> CO₂ (g) + 2H₂O (l)

Analysis of the above equations tells us that:

This equation says that you need twice as many molecules of oxygen as you do of methane. According to Avogadro's Law, this means that you will need the volume of oxygen as the volume of methane. So, if you have to burn 1 litre (1 dm₃) of methane, you will need 2 litres (2 dm³) of oxygen. Similarly, for every molecule of methane you burn, you will get a molecule of carbon dioxide formed. Therefore every litre of methane will give 1 litre of carbon dioxide, because 1 litre of methane and 1 litre of CO2 contain the same number of molecules.

EXAMPLE 2

Carbon monoxide burns in oxygen to give carbon dioxide:

2CO (g) + O2 (g) --> 2CO2 (g)

If you burned 1 litre of carbon monoxide, you would only need half of that volume of oxygen (0.5 litres) because you only need half as many molecules. Because of the number of molecules of carbon monoxide burnt, the volume of CO₂ produced will be 1 litre.

EXAMPLE 3

What volume of air is needed to burn completely 100 cm3 of propane?

C₃H₈ (g) + 5O₂ (g) --> 4H₂O (l)

You need 5 times as many molecules of oxygen as you do of propane, and so you need 5 times the volume. The volume of oxygen needed is therefore 500cm³. However, you are asked about the volume of air, not of oxygen. Air is only approximately 1/5 oxygen - and so you need 5 times more air than you would need oxygen. The amount of air needed is 2500 cm³.