This law was given by, a French chemist , Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight.
Proust worked with two samples of curpic carbonate- one of which was of natural origin and the other was synthetic one . He found that the composition of elements present in it was for both the samples as shown below:
Thus, irrespective of the source, a given compound always contains same elements in the same proportion. The validity of this law has been confrmed by various experiments. It is sometimes also reffered to as Law of constant composition.
Limitation:
This law is not applicable if the compound is formed from different isotopes of an element . The two isotopes of carbon C-12 and C-14 form carbondioxide `C^(12)O_(2)` and `C^(14)O_(2)` . The ratio of C : O is 12 : 32 and 14 : 32 respectively .It is not a constant ratio.
0.5 G silver is dissolved in excess of nitric acid. This solution is treated with excess of NaCl solution when 0.66 g AgCl is formed. One gram metallic silver wire is heated in dry `Cl_(2)` ,1.32 g AgCl is formed . Show that these data confirm the law of constant proportion.

Answer

`%Ag=(0.5)/(0.66)xx100=75.75`
`%Ag=(1)/(1.32)xx100=75.75`
In both `AgCl,%` Ag is same,