Law of constant composition
In any given chemical compound, the elements always combine in the same proportion with each other. This is the law of constant composition.
The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a ratio. If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen. This mass proportion will be the same for any water molecule.
This does not mean that hydrogen and oxygen always combine in a ratio to form . Multiple proportions are possible. For example, hydrogen and oxygen may combine in different proportions to form rather than . In , the ratio is and the mass ratio of hydrogen to oxygen is . This will be the same for any molecule of hydrogen peroxide.
Volume relationships in gases
In a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure. This relationship is also known as Gay-Lussac's Law.
For example, in the reaction between hydrogen and oxygen to produce water, two volumes of react with 1 volume of to produce 2 volumes of .
In the reaction to produce ammonia, one volume of nitrogen gas reacts with three volumes of hydrogen gas to produce two volumes of ammonia gas.