What is Molar Volume?
We already discussed in another article (‘…mole concept…‘ ) that a mole of any particular substance has unique mass, and the article also shows how to use mass and moles in calculations.
And that works great for things we can easily measure the mass, easily weigh. Like solids and liquids.
Gases have mass too, of course. You can use mass and moles in exactly the same way, just so long as you know the mass of the gaseous substance under consideration.
But gases are not so easy to weigh. You can’t just pour them into a flask on a balance, or spatula it into a beaker.
That’s where molar volume comes in. A mole of gas at a particular temperature and pressure has a particular volume, and that is its molar volume.
And that volume is the same for a mole of any gas under those conditions.
Listen to “How To Use Moles for Gases – Molar Volume” (8 mins listen)
Molar Volumes to Remember
You’ll be glad to know that you don’t need to remember the molar volume of gas at every possible temperature and pressure combination.
Actually, you can work it out quite easily and I will show you how later in this article.
However, there are two sets of conditions that you should commit to memory because they come up time and again in your exam questions or assignments.
The conditions are ‘standard temperature and pressure‘ (STP) and ‘room temperature and pressure‘ (RTP).
Standard Temperature and Pressure (STP)
This refers to temperature of 0°C (273K) and pressure of 1 atmosphere.
The molar volume of any gas at STP is 22.4 dm³ (22.4 L).
“The volume of any gas at STP is 22.4 litres.”
Room Temperature and Pressure (RTP)
RTP refers to temperature of 25°C (298K) and pressure of 1 atmosphere.
The molar volume of any gas at RTP is 24.0 dm³ ( 24.0 L).
“The volume of any gas at RTP is 24 litres.”
How To Calculate Molar Volume at Different Conditions
It’s really easy to calculate the volume of a mole of gas for any temperature and pressure. We just use the Ideal Gas Equation.
The Ideal Gas Equation: PV = nRT
P = pressure (Pa)
V = volume (m³)
n = number of moles
R = gas constant (8.31 J K-1 mol-1)
T = temperature (K)
The next step is to rearrange the equation to isolate volume. This gives:
V = nRT/P
Simply put in the values for the temperature and pressure you are using, and the values of n and R.
Tip: Take care to use the correct units. The most common mistake I see is not converting the volume to the correct unit – remember that 1m³ = 1000 litres or 1000 dm³ .
Another common mistake is using atmospheres instead of pascals for the pressure term.