A closer look at some gases
Imagine five very large flasks, each with a volume of 24dm3. Each is filled with different gas. Each gas is at room temperature and pressure, or rtp.
(We take room temperature and pressure as the standard conditions for somparing gases; rtp is 20 degrees celcius and 1 atmosphere.
If you weighed the gas in the five flasks, you would discover something amazing. There is exactly 1 mole of each gas!
So we can conclude that:
1 mole of every gas occupies the same volume, at the same temperature and pressure. At the room temperature and pressure this volume is 24dm3.
This was discovered by Avogrado, in 1811. So its often called Avogrado's Law. It does not matter whether a gas exists as atoms or molecules, or whether its atoms are large or small. The law still holds.
The volume occupied by 1 mole of a gas is called its molar volume.
The molar volume of a gas is 24dm3 at rtp.
Use the calculation triangle:
Calculating gas volumes from moles and grams
Avogrado's Law makes it easy to work out the volumes of gases.
Example 1 What volume does 0.25 moles of a gas occupy at rtp?
1 mole occupies 24dm3 so
0.25 moles occupies 0.25 x 24dm3 = 6dm3
so 0.25 moles of any gas occupies 6 dm3 (or 6000cm3) at rtp.
Example 2 What volume does 22g of carbon dioxide occupy at rtp?
Mr of carbon dioxide = 44, so
44g = 1 mole, so
22g = 0.5 mole
so the volume occupied = 0.5 x 24dm3 = 12 dm3.
no of moles x 24dm3
Volume at rtp (dm3)