CALCULATIONS INVOLVING HOMOGENEOUS GAS EQUATIONS

(i.e. where all the chemicals on both sides of the equation are gases.)

E.g.         N2 (g)      +              3H3 (g)                    =              2NH3 (g)

Calculations with such equations can often be simplified by applying GAY - LUSSAC'S LAW OF COMBINING VOLUMES (1808)

This states:

The volumes of gases taking part in a chemical change, either as reactants or as products, bear a simple numerical relation to one another, provided that all the measurements are made under the same conditions of temperature and pressure.

This can be simply illustrated as follows:

 

N2 (g)

+

3H2 (g)

=

2NH3 (g)

Equation shows

1 mol N2

+

3 mol H2

Gives

2 mol NH3

Equation shows

1 Volume N2

+

3 volumes H2

Gives

2 Volumes NH3

So for example

10cm3 N2

+

30cm3 H2

Gives

20cm3 NH3

 

Now try the following calculations.

Qu. 1

In the reaction                      N2            +              3H2          =              2NH3

20cm3 nitrogen and 70cm3 hydrogen were reacted.

Considering that all measurements are made under identical conditions of temperature and pressure.

a.        Calculate the reacting volume (volumes actually reacting)

b.       The volume of NH3 produced

c.        The final volume (i.e. volume NH3 + any unused reactants.)

 

                Gareth Mc Bride: Chemistry Department: Loreto College, Coleraine.