What is the law of proportions?

1 Answer
Aug 3, 2014

There are two laws regarding law of proportions .

  • By French chemist Joseph Proust
  • ' A chemical compound always contains exactly the same proportion of elements by mass .'
  • Proust worked with 2 samples of same compound Cupric carbonate ( one was it's natural form & the other one was synthetic . )
  • It was observed that the percentage composition of each atom was same in both the sources .
  • Thus irrespective of the source , a chemical compound always contains the elements in a fixed ratio by mass .
  • Examples : Pure water obtained from any source always contains hydrogen & oxygen combined in a ratio of 1:8 by mass .


  • By Dalton
    - 'When 2 elements combine with each other to form 2 or more than 2 compounds , the masses of one of the elements which combine with the fixed mass of the other , bear a simple whole number ratio ."
  • Example - C & O combine with each other to form 2 types of compounds : carbon monoxide & carbon dioxide .
  • In CO : 12 parts by mass of C reacts with 16 parts by mass of O
  • In CO2 : 12 parts by mass of C reacts with with 32 parts by mass of O2 .
  • Ratio of masses of O2 which combine with fixed mass of C bears a simple whole no. ratio i.e 16:32 = 1 :2

Difference between these 2 laws is that the first one is applicable for same compound extracted from different resources while the 2nd law is applicable when the reacting species combine to form 2 or more than 2 compounds .