What is Avogadro’s law?

Short Answer

Avogadro’s law states that equal volumes of all gases contain an equal number of molecules when kept at the same temperature and pressure. This means that the amount of gas (in moles) is directly proportional to its volume under the same conditions.

For example, 1 mole of any gas—such as oxygen, hydrogen, or nitrogen—occupies the same volume (22.4 liters) at standard temperature and pressure (STP). This law helps in understanding the relationship between gas volume and number of particles.

Detailed Explanation :

Avogadro’s Law

Avogadro’s law is a fundamental gas law that explains the relationship between the volume of a gas and the number of molecules (or moles) present in it. It was proposed by the Italian scientist Amedeo Avogadro. According to this law, when gases are kept at the same temperature and pressure, their volumes depend only on the number of molecules they contain, not on the type of gas.

This law shows that gases with different chemical properties behave similarly when temperature and pressure are constant.

Statement of Avogadro’s Law

The law can be stated as:

“Equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules.”

Another way to state it is:

“The volume of a gas is directly proportional to the number of moles at constant temperature and pressure.”

Mathematically,
V ∝ n
or
V / n = constant

Here,
V = volume of the gas
n = number of moles

Explanation of Avogadro’s Law

To understand Avogadro’s law, imagine two balloons filled with different gases, such as hydrogen and oxygen, at the same temperature and pressure. Even if the gases are different, if both balloons have the same volume, they contain the same number of molecules.

This is because:

  • Gas molecules are very far apart
  • Their sizes do not affect volume significantly
  • Their chemical nature does not matter
  • Only the number of particles decides how much space they occupy

Therefore, Avogadro’s law works for all gases and is considered a universal law.

Relation with the Ideal Gas Equation

The ideal gas equation is:

PV = nRT

If pressure (P) and temperature (T) remain constant, then:

V ∝ n

This matches exactly with Avogadro’s law. Thus, Avogadro’s law is built into the ideal gas equation and supports the idea that gases behave ideally under common conditions.

Importance of Avogadro’s Number

Avogadro’s law is closely linked with Avogadro’s number, which is:

6.022 × 10²³ molecules per mole

This means:

  • 1 mole of any gas contains the same number of molecules
  • This helps in converting between mass, moles, and volume

Avogadro’s number makes it possible to calculate how many molecules are present in a given volume of gas.

Molar Volume of a Gas

Avogadro’s law leads to an important concept called molar volume.

At STP (Standard Temperature and Pressure):
Temperature = 0°C or 273 K
Pressure = 1 atm

1 mole of any gas occupies 22.4 liters.

So, hydrogen, oxygen, carbon dioxide, chlorine, and any other gas will have the same volume for 1 mole at STP.

This volume is known as the molar volume of a gas.

Graphical Representation

A graph of volume (V) versus number of moles (n) is a straight line passing through the origin.
This shows that volume increases evenly as the number of moles increases.

If the number of gas molecules is doubled while keeping temperature and pressure constant, the volume also doubles.

Examples of Avogadro’s Law in Daily Life

  1. Inflating a Balloon

When you blow more air into a balloon, the number of molecules increases, so the volume increases.

  1. Breathing

When we breathe in, more air molecules enter the lungs, increasing their volume.
When we breathe out, volume decreases.

  1. Cooking Gas Cylinders

When gas is released, the number of gas molecules decreases, and so does the available volume inside the cylinder.

  1. Football Pumping

Pumping more air into a football increases the number of moles, causing the ball to expand.

  1. Industrial Gas Storage

Industries use Avogadro’s law to store and measure large amounts of gases accurately.

Applications of Avogadro’s Law

Avogadro’s law is used in:

  • Chemical reactions involving gases
  • Determining molar volume
  • Calculating number of molecules
  • Balancing chemical equations
  • Understanding gas mixture behavior
  • Designing gas cylinders and storage systems

It forms the basis of the mole concept used in chemistry and physics.

Why Avogadro’s Law Works Best at Low Pressure

At low pressure:

  • Gas molecules are far apart
  • Intermolecular forces are negligible
  • Gas volume depends mainly on number of molecules

Thus, the law is most accurate under low-pressure, high-temperature conditions.

Conclusion

Avogadro’s law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. It shows the direct relationship between gas volume and number of moles. This law helps define molar volume, explains gas behavior, and simplifies calculations in chemistry and physics. Avogadro’s law is essential in understanding the molecular nature of gases and forms a key part of the ideal gas model.