Laws of Proportions

Key Questions

  • Answer:

    Because it defines that all water molecules are #H_2O#, for example.


    The law of definite proportions dictates that a name is always associated with a specific ratio of elements found in a chemical compound. If the ratio of elements is different from that specific ratio then it is not the same compound and therefor has a different name.

  • The law of definite proportions states that a compound always contains exactly the same proportion of elements by mass.

    This means that the composition of a compound does not depend on its source or its method of preparation.

    In water, for example, the mass of oxygen must always have the same ratio to the mass of hydrogen (mass ratio is #"O:H = 7.94:1"#).

    To tell if compounds containing the same elements are the same or different, we can determine the ratios of their masses.


    Compound 1 contains 15.0 g of hydrogen and 120.0 g oxygen. Compound 2 contains 2.00 g of hydrogen and 32.0g oxygen. Are the compounds the same or different?


    Compound 1: #"Mass ratio of O:H" = (120.0 color(red)(cancel(color(black)("g"))))/(15.0 color(red)(cancel(color(black)("g")))) = 8.00#

    Compound 2: #"Mass ratio of O:H" = (32.0 color(red)(cancel(color(black)("g"))))/(2.00 color(red)(cancel(color(black)("g")))) = 16.0#

    The mass ratios are not the same. The compounds are different.

  • 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 .