GAY LUSSAC'S LAW OF COMBINING VOLUME:

At given temperature and pressure the volumes of all gaseous reactants and products bear a simple whole number ratio to each other.

For example:

I.e. one volume of hydrogen reacts with one volume of chlorine to form two volumes of HCl gas. I.e. the ratio by volume which gases bears is 1:1:2 which is a simple whole number ratio.

Similarly other examples are:

ILLUSTRATIVE EXAMPLE (14):  In the reaction

the ratio of volumes of nitrogen, hydrogen and ammonia is 1: 3 : 2. These figures illustrate the law of:

                         (A) Constant proportions                               (B) Gay-Lussac

                        (C) Multiple proportions                                 (D) reciprocal proportions

SOLUTION:  The above ratio of 1: 3: 2 illustrates the Gay-Lussac law of combining volume. 

                          Hence (B) is correct.

ILLUSTRATIVE EXAMPLE (15): How much volume of oxygen will be required for complete combustion of 40 ml of acetylene (C₂H₂) and how much volume of carbon dioxide will be formed? All volumes are measured at NTP.

SOLUTION:

So for complete combustion of 40 ml of acetylene, 100 ml of oxygen are required and 80 ml of CO₂ is formed.