The volume occupied by one mole of any gas at standard temperature(0^o) and pressure(1 atm) is 22.4 litres. This volume is a close approximation and different gases vary slightly.

The molar volume of any gas at S.T.P. is 22.4L .The mole of any gas can therefore be found using the expression on the right. Lets do some examples.

Example 1
Calculate the number of mole of hydrogen gas that occupies a volume of 2.24L at S.T.P.
=> mole = 2.24/22.4 = 0.1

Example 2
Calculate the mass of oxygen gas(O₂) that expands to a volume of 33.6L at 0^oC and 1 atm pressure. Atomic mass O =16.
The conditions are obviously at S.T.P. so we can use the formula above.
Step 1) Find the number of mole of oxygen gas.
=> mole = 33.6/22.4 = 1.5
Step 2) Find the mass of oxygen gas represented by 1.5 mole.
=> mass = mole X molar mass = 1.5 X 32 = 48 grams

Example 3
12 grams of hydrogen gas(H₂) is placed in a piston at 0^oC and 760mmHg pressure. What is the volume to which the piston expands? Atomic mass H=1
The conditions are S.T.P. temperature is 0^oC and pressure is 760mmHg = 1atm.
Step 1) Convert the expression above to make it equal to V
=> V = n X 22.4L
Step 2) Calculate the mole of hydrogen
=> mole = mass / molar mass = 12 /2 = 6 mole
Step 3) Calculate the volume
V = 6 x 22.4L = 13.4L

Try these exercises.
1) What is the volume occupied by 3.5 mole of nitrogren gas as S.T.P?
Answer
2) What mass of nitrogen gas(N₂) occupies a volume of 11.2L at 0^oC and 760mmHg pressure. (Atomic mass N=14)
Answer
3) How many mole of helium gas should be placed in a piston so that it expands to fill a volume of 56.8L at S.T.P.?
Answer
4) 35 grams of an unknown gas occupies a volume of 11.2 litres at S.T.P. Scientists suspect nitrogen(N₂), oxygen(O₂), chlorine(Cl₂) or hydrogen(H₂) gas. Which one is it likely to be?
Atomic mass H=1,O=16,Cl=35.5,N=14.
Answer