## Gay Lussac’s Law of Combining Volumes

Gay Lussac’s Law of Combining Volumes states that when gases react, they do so in volumes which bear a simple ratio to one another, and to the volume of the product(s) formed if gaseous, provided the temperature and pressure remain constant.

The law explains experimental facts about how gaseous atoms combine. Example:

For the reactions:

(i) N2(g) + 3H2(g) → 2NH3(g)

1 vol.    3 vols.    2 vols.

1 volume of nitrogen combines with 3 volumes of hydrogen to form 2 volumes of ammonia.

(ii) 2H2(g) + O2(g) → 2H2O(g)

2 vols.   1 vol.     2 vols.

2 volumes of hydrogen combine with 1 volume of oxygen to form 2 volumes of steam.

(iii) Cl2(g) + H2(g) → 2HCl(g)

1 vol.    1 vol.    2 vols.

1 volume of chlorine gas combines with 1 volume of hydrogen to form 2 volumes of hydrochloric acid.

Question: Consider the reaction: 2H2(g) + O2(g) → 2H2O(g)

(a). What volume of steam is formed from 20 cm3 of hydrogen and 20 cm3 of oxygen mixed together?

(b). What gas(s) is in excess, and by what amount?

Solution: (a). The ratio of their volumes is 2 vols. : 1 vol. → 2 vols.

20 vols. : 10 vols.  → 20 vols.

That means, 20 cm3 of hydrogen will combine with 10 cm3 of oxygen to form 20 cm3 of steam.

(b). Oxygen is in excess by 10 cm3.