What is Avogadro’s law?

Short Answer

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

In simple words, if the number of gas molecules increases, the volume also increases, and if the number of molecules decreases, the volume decreases. This law helps us understand gas behaviour and is useful in calculating the number of particles in a gas sample.

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 particles it contains. Named after the Italian scientist Amedeo Avogadro, this law states that equal volumes of different gases contain the same number of molecules if temperature and pressure are kept constant. The law is very important for understanding how gases behave and is widely used in chemistry, especially in studying gases and performing gas calculations.

According to Avogadro’s law, the volume of a gas is directly proportional to the number of moles (amount of gas). This means that if the number of gas particles increases, the volume also increases. If the number of gas particles decreases, the volume decreases. This direct relationship helps in predicting how gases will behave when their quantity changes while temperature and pressure stay the same.

Direct Relationship Between Volume and Number of Molecules

The main idea of Avogadro’s law is that volume increases with the number of gas molecules. When more particles are added to a container at constant pressure and temperature, the particles need more space to move freely, so the volume increases. When some particles are removed, the volume decreases because fewer particles require less space.

This is written as:
V ∝ n
Where:

  • V = volume of gas
  • n = number of moles

This means volume and number of moles rise and fall together. The relationship holds true only if temperature and pressure remain unchanged.

Explanation Based on Particle Motion

To understand why this law works, we must think about how gas particles behave. Gas molecules move freely and occupy all available space. The movement and collisions of these particles decide how much space the gas occupies.

When more molecules are added:

  • More collisions occur with container walls.
  • The gas needs more space to keep the pressure constant.
  • As a result, the volume increases.

When fewer molecules are present:

  • Fewer collisions take place.
  • Less space is needed to maintain constant pressure.
  • The volume decreases.

This shows that the number of molecules directly affects the space required by the gas.

Mathematical Expression of Avogadro’s Law

The mathematical form of Avogadro’s law is:
V₁ / n₁ = V₂ / n₂

Where:

  • V₁ = initial volume
  • n₁ = initial number of moles
  • V₂ = final volume
  • n₂ = final number of moles

This equation shows that the ratio of volume to moles remains constant when temperature and pressure are unchanged.

Equal Volumes Contain Equal Numbers of Molecules

A very important statement of Avogadro’s law is:
Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.

This means that a litre of oxygen, a litre of hydrogen, and a litre of nitrogen contain the same number of molecules if measured under identical conditions. This is true despite the differences in the types or masses of the gas particles.

This concept helps chemists compare gases easily and understand how they react in fixed proportions.

Importance in Calculating Moles and Molar Volume

Avogadro’s law helps in calculating the number of particles in a gas sample. At standard temperature and pressure (STP), one mole of any gas occupies 22.4 litres. This is known as molar volume. It is directly based on Avogadro’s law.

This makes it easy to:

  • calculate moles from volume,
  • find volume from moles,
  • compare different gases,
  • understand gas reactions and stoichiometry.

Everyday Examples of Avogadro’s Law

Avogadro’s law can be seen in real-life situations:

  1. Inflating a Balloon:
    Adding more air (more gas particles) increases the volume of the balloon.
  2. Filling Tyres:
    More air particles increase tyre volume and pressure when temperature is constant.
  3. Airbags in Cars:
    When the airbag inflates, gas molecules are rapidly produced, increasing its volume instantly.
  4. Breathing:
    When lungs take in more air, their volume increases because more gas particles enter.
  5. LPG Cylinders:
    Adding more gas molecules increases the volume the gas would occupy if it were not compressed.

These examples show how an increase in gas molecules increases the required space.

Importance in Science and Industry

Avogadro’s law is used in many scientific and industrial applications:

  • determining molar masses of gases,
  • studying chemical reactions involving gases,
  • calculating volumes in laboratory experiments,
  • designing gas storage systems,
  • making meteorological predictions.

It also plays a key role in the ideal gas equation and the combined gas law.

Conclusion

Avogadro’s law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, and that gas volume is directly proportional to the number of moles. This law helps explain how gases expand when more particles are added and shrink when particles are removed. It is essential for understanding gas behaviour, calculating molar volume, and studying gas reactions in chemistry.