Molar Volume

Molar volume is the volume occupied by one mole of any gas at standard temperature and pressure (STP), which is a standard value of 22.4 litres. One mole contains Avogadro's number, or 6.02 into 10 to the power of 23 particles, meaning that this exact count of gas atoms or molecules occupies 22.4 litres under these conditions.

What STP means

The volume of a gas changes whenever the surrounding temperature and pressure change. Standard temperature and pressure, shortened as STP, keeps gas at atmospheric pressure and a temperature of 0 degrees Celsius. At this temperature and pressure, gas particles move more slowly than at room temperature, allowing the gas to hold a fixed, measurable volume.

Finding particles from a gas volume

• Divide the known gas volume by the molar volume of 22.4 litres to convert the volume into moles.
• Multiply the resulting moles by Avogadro's number, which is 6.02 into 10 to the power of 23 particles.
• For a 20-litre container at STP, dividing 20 by 22.4 gives 0.89 moles of gas.
• Multiplying 0.89 by 6.02 into 10 to the power of 23 gives 5.36 into 10 to the power of 23 particles inside the container.

Finding gas volume from particle count

• Divide the given number of gas particles by Avogadro's number to find the number of moles.
• Multiply the number of moles by the standard molar volume of 22.4 litres.
• For example, dividing 2.6 into 10 to the power of 22 particles by Avogadro's number gives 0.043 moles.
• Multiplying 0.043 moles by 22.4 litres shows that those particles take up 0.96 litres, which is almost one litre.

Summary of conversions at STP

• To find particle count from volume, divide volume by molar volume and multiply by Avogadro's number.
• To find volume from particle count, divide particle count by Avogadro's number and multiply by molar volume.
• Both methods work because one mole of any gas always takes up 22.4 litres at STP.