Question

What is the law of proportions?

Answer 1

There are two laws regarding law of proportions .

• LAW OF CONSTANT OR DEFINITE 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 .

• LAW OF MULTIPLE PROPORTIONS

• 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 .