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The law deals with the relation between the mass of the reactants and products in physical changes or chemical reactions. The law was established by a French Chemist Antoine Lavoisier. He brought a revolution in the field of chemistry. Along with co-workers he gave the modern system of nomenclature of chemical substances and also won a noble prize. According to the law :
In all physical changes and chemical reactions , the total mass of the products is the same as the total mass of the reactants.

The law was given by Dalton in 1803. It states that
If two elements combine to form two or more compounds, the weights of one the elements which combine with a fixed weight of the other in these compounds, bear a simple whole number ratio by weight .

The relationship between the masses of the elements when more than two such elements combine between themselves was studied by Ritcher. It was stated in the form of Law of Reciprocal Proportions as follows :
The ratio in which two elements combine separately with a fixed weight of the third element is either the same or is a simple whole number multiple of the ratio in which these elements combine with each other.

From the study of the Gay Lussac’s Law of combining volume , we have concluded that the gases always combine in simple whole number ratios of volumes. It was left to an Italian chemist Amedeo Avogadro to establish a relationship between the volume of a gas and the corresponding number of molecules under given set of conditions of temperature and pressure. The hypothesis, known as Avogadro’s hypothesis, may be stated as :
Under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.

We know that the smallest portion of matter is atom which may or may not exist independently. But as it is extremely small in size, it is not possible to isolate a single atom and weight it. Therefore, a single atom cannot be weighed. The mass of an atom of hydrogen is 1.672 × 10–24 g. It is so small that a gram is very big unit to express the same. In order to solve this problem, it was suggested that the mass of an atom should be expressed as relative mass. It could be done by fixing some standard of a mass and then comparing the mass of the atoms of any element relative to it. It is this relative mass which is now known as the atomic mass unit (a.m.u.) or as unified mass (u).

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