According to modern researches, it is stated that mass can be converted into energy. Some mass has been lost due to formation of some energy in each reaction. According to Einstein, mass and energy are related as `E = mc^2`

where, m = the mass of substance, c = velocity of light (`3xx10^8 m//s`) and E = energy

But in chemical reactions energy released or absorbed is too less, therefore, the change in mass (decrease or increase) taking place becomes negligible. These changes are clearly seen in nuclear reactions.

This law was stated by French chemist Joseph Proust.

Statement : "A sample of a pure chemical compound always consists same elements combined together in the same definite proportions by mass, whatever be its source".

Example : It is found by the analysis of water (taken from various places like river, falls and wells) that in each sample of water, the ratio of hydrogen and oxygen is `2 : 16` or `1 : 8` by mass.

This law was proposed by John Dalton.

Statement : "An element may form more than one compound with another element. For a given mass of an element, the masses of other elements (in two or more compounds) come in the ratio of small integers."

Example : In `NH_3, 14 g` of nitrogen requires `3 g` of hydrogen and in hydrazine `(N_2H_4) , 14 g` of nitrogen requires `2 g` of hydrogen. Hence, fixed mass of nitrogen requires hydrogen in the ratio `3 : 2` in two different compounds (`3 : 2` is a simple ratio). Thus, this is in agreement with "law of multiple proportions".

This law was proposed by Richter in 1972. This law is known as the law of equivalent proportions or law of combining weights.

Statement : "When two different elements combine separately with the same weight of a third element the ratio in which they do so will be the same or some simple multiple of the ratio in which they combine with each other".

Example : Carbon and sulphur react separately with oxygen (third element) to give carbon dioxide (`CO_2`) and sulphur dioxide (`SO_2`). They also react together to form carbon disulphide (`CS_2`).

Now, in carbon dioxide 12 parts by weight of carbon combines with 32 parts by weight of oxygen and in sulphur dioxide (`SO_2`) 32 parts by weight of sulphur combines with 32 parts by weight of oxygen. Ratio of weight of carbon and sulphur which combine with fixed weight (32 parts) of oxygen is ` = 12 : 32` or `3 : 8` .............(i)

In carbon disulphide, 12 parts of weight of carbon reacts with 64 parts by weight of sulphur. Hence, ratio of weight of carbon and sulphur in which they combines to form `CS_2` is ` = 12 : 64` or `3 : 16 ` ............(ii)

Ratio (i) and (ii) are related to each other as follows : `3 : 8` and `3 : 16` or `3 : 3` and `8 : 16` and `1 : 2`.

Thus, this is an agreement with "law of reciprocal proportions."

This law was proposed by Gay-Lussac. This law gives the relation between volumes of reactants and products during chemical reactions.

Statement : "Under the same conditions of temperature and pressure whenever gases react together, the volume of the reacting gases as well as products bears a simple whole number ratio."

Example : One volume of hydrogen react with one volume of chlorine to give two volumes of hydrogen chloride (gas).

`underset(1 vol)H_2+underset(1 vol)(Cl_2) → underset(2 vol)(2HCl)`

Hence, volume ratio of `H_2 : Cl_2 : 2HCl` is `1 : 1 : 2`