French
chemist, Antoine Laurent Lavoisier (1743-1794) experimentally
showed that matter can neither be created nor destroyed
in a chemical reaction. This experimental finding was known
as law of conservation of mass. In fact, this could be possible
due to precise measurement of mass by Lavoisier. Law of
conservation of mass helped in establishing the law of definite
composition or law of constant proportions. This law states
that any sample of a pure substance always consists of the
same elements combined in the same proportions by mass.
For instance, in water, the ratio of the mass of hydrogen
to the mass of oxygen is always 1:8 irrespective of the
source of water. Thus, if 18.0 g of water are decomposed,
2.0 g of hydrogen and 16.0 g of oxygen are always obtained.
Also, if 2 g of hydrogen are mixed with 16.0 g of oxygen
and mixture is ignited, 18.0 g of water are obtained after
the reaction is over. In the water thus formed or decomposed,
hydrogen to oxygen mass ratio is always 1:8. Similarly in
ammonia (NH3), nitrogen and hydrogen will always react in
the ratio of 14:3 by mass.
John Dalton thought about the fact that an element may form
more than one compound with another element. He observed
that for a given mass of an element, the masses of the other
element in two or more compounds are in the ratio of simple
whole number or integers. In fact this observation helped
him in formulation of his fundamental theory popularly known
as Dalton’s ‘Atomic theory’ which is discussed
in Section 2.3. Let us take two compounds of nitrogen and
hydrogen : (i) ammonia (NH3) and (ii) hydrazine (N2H4).
In ammonia, as discussed above, 3.0 g of hydrogen react
with 14 g of nitrogen. In hydrazine, 4.0 g of hydrogen react
with 28 g of nitrogen or 2.0 g of hydrogen reacts with 14.0
g of nitrogen. It can be seen that for 14 g of nitrogen,
we require 3.0 g of hydrogen in NH3 and 2.0 g of hydrogen
in hydrazine (N2H4). This leads to the ratio
That is, masses of hydrogen which combine with the fixed
mass of nitrogen in ammonia and in hydrazine are in the
simple ratio of 3:2. This is known as law of multiple proportions.
