What is molar volume of a gas at STP?

Short Answer

The molar volume of a gas at STP is the volume occupied by one mole of any gas when it is kept at Standard Temperature and Pressure (STP). At STP conditions—temperature of 0°C (273 K) and pressure of 1 atm—one mole of any ideal gas occupies 22.4 litres.

This value is useful because it allows chemists to easily convert between moles and volume of gases in stoichiometric calculations. It also helps compare different gases under the same standard conditions.

Detailed Explanation

Molar Volume of a Gas at STP

The molar volume of a gas at STP refers to the specific volume taken up by exactly one mole of any gas when it is placed under standard conditions of temperature and pressure. These standard conditions were chosen so that chemists worldwide can use a common reference point for calculations involving gases.

STP conditions are defined as:

  • Temperature = 0°C (273 K)
  • Pressure = 1 atmosphere (1 atm)

Under these conditions, one mole of any ideal gas occupies a volume of 22.4 litres. This value is known as the standard molar volume.

The idea of molar volume allows chemists to work with gases in a simple and predictable way, using the fact that all gases behave similarly at the same temperature and pressure if they are considered ideal.

Why molar volume is the same for all gases at STP

According to Avogadro’s Law, equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. This means:

  • 1 mole of oxygen gas contains the same number of molecules as
  • 1 mole of hydrogen gas or
  • 1 mole of nitrogen gas

Because one mole of any gas contains 6.022 × 10²³ molecules, all gases must have the same volume if conditions are the same. That common volume is 22.4 L at STP.

This is true for ideal gases and nearly true for real gases, as long as temperature and pressure are moderate.

How the molar volume is used in calculations

The molar volume helps chemists convert between moles and gas volume easily. At STP:

  • 1 mole of gas = 22.4 L
  • 2 moles of gas = 44.8 L
  • 0.5 moles of gas = 11.2 L

This relationship is extremely useful in stoichiometry, where gases are often involved in reactions.

Example:
If a reaction produces 3 moles of CO₂ gas at STP, the total volume is:
3 × 22.4 L = 67.2 L

This simple conversion saves time and makes gas calculations easier.

Relation to the Ideal Gas Law

The molar volume can also be calculated using the Ideal Gas Equation:
PV = nRT

At STP:

  • P = 1 atm
  • T = 273 K
  • n = 1 mole
  • R = 0.0821 L·atm/mol·K

Substituting values:

V = nRT / P
V = (1 × 0.0821 × 273) / 1
V ≈ 22.4 L

This mathematical result matches the standard molar volume.

Applications of molar volume

Molar volume is widely used in:

  1. Stoichiometry involving gases

It allows easy conversion between gas amount and volume.

  1. Industrial gas production

Factories calculate gas storage and transport needs using molar volume.

  1. Environmental science

Air pollutants like CO₂ and NO₂ are measured using molar volume relationships.

  1. Laboratory experiments

Gas volumes collected over water or in syringes can be converted to moles.

  1. Combustion calculations

Fuel burning in engines and stoves involves gas volumes.

Difference between STP and other gas conditions

STP is only one set of standard conditions. There are other conditions like SATP (Standard Ambient Temperature and Pressure), where the molar volume is slightly different.

  • STP: 22.4 L/mol (0°C, 1 atm)
  • SATP: 24.8 L/mol (25°C, 1 atm)

Chemists must know which standard is being used during calculations.

Why 22.4 L is an approximate value

The value 22.4 L is based on the behaviour of an ideal gas. Real gases behave almost ideally at:

  • Low pressure
  • Moderate temperature

But at very high pressure or very low temperature, gases behave differently, and deviations occur. However, for most practical purposes, 22.4 L/mol is accurate enough.

Conclusion

The molar volume of a gas at STP is the volume occupied by one mole of any gas under standard conditions of 0°C and 1 atm pressure. This volume is 22.4 litres and is the same for all ideal gases. Molar volume is a key concept in gas stoichiometry because it enables easy conversion between volume and amount of gas. It is widely used in laboratory experiments, industrial processes, combustion calculations, and environmental analysis.