Molar volume
The molar volume is the volume occupied by one mole of any gas. The same value is obtained for all gases at the same temperature and pressure.
The value of the molar volume will be different for different temperatures and pressures and it is measured in litres per mole \((l\,mol^{-1})\).
As one mole of every gas will occupy the same volume at a given temperature and pressure, we can use volumes and the molar ratio to calculate volumes of reactants or products.
Consider the following reaction:
Ammonia (NH3), which is both a useful and profitable compound, can be produced in the chemical industry from the relatively cheap reactants nitrogen and hydrogen. The balanced equation for this process is:
\(N_{2} (g) + 3H_{2} (g) \rightarrow 2NH_{3} (g) \)
Example
When 400 cm3 of nitrogen reacts with excess hydrogen, calculate the volume of ammonia that will be produced.
Looking at the balanced equation, we can see that one mole of nitrogen reacting results in the formation of two moles of ammonia.
\(N_{2} (g) + 3H_{2} (g)\rightarrow 2NH_{3} (g) \)
\(1\,\,mole\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,2\,\,moles \)
The question does not give us information about the masses involved, but since the products and reactants are at the same temperature and pressure, we know that one mole of each product and reactant will occupy the same volume.
The molar ratio of nitrogen to ammonia from the balanced equation is 1:2. This means that if 400 cm3 of nitrogen are used then we will form double that volume of ammonia.
\(N_{2} (g) + 3H_{2} (g)\rightarrow 2NH_{3} (g) \)
\(400cm^{3}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,800cm^{3} \)
