Hello steemians, this is the second post of series of posts on "laws of chemical combination". I have already posted about law of conservation of mass. In this article, I am going to write about law of definite proportion.

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Law of definite proportion

This law was first put forward by French chemist Louis Proust in 1799 AD. It states that, “In same chemical compound, same elements are present which are combined in fixed weight composition irrespective of source and mode of formation.”

This law can be explained by following illustrations.

Illustration 1

Carbon dioxide is found in all countries. If the pure sample of carbon dioxide is taken from different countries, its chemical formula is same CO2. The carbon and oxygen combine in the fixed ratio 12:32 or 3:8 irrespective of country.
This verifies law of definite proportion.

Illustration 2

Carbon dioxide can be prepared by various methods as shown:
a. Calcium Carbonate Calcium Oxide + Carbon dioxide

b. Calcium carbonate + Hydrochloric acid Calcium chloride + Carbon dioxide + water

c. Carbon + Oxygen Carbon Dioxide

If the samples of carbon dioxide formed by different preparation methods are checked, there would be definite composition of carbon and oxygen to form carbon dioxide. i.e in the ration 3:8 by weight.
There are the conditions where law of definite proportion doesn’t hold good. They are as follows

a. If different isotope of element is used.
If is used, H2O has hydrogen to oxygen ratio 1:8 by weight
If is used, H2O has hydrogen to oxygen ratio 1:16 by weight
So, different isotopes of same element give different proportion in chemical combination

b. There are certain compounds called non stoichiometric compounds or Berthollite compounds, which don’t follow law of constant proportions. Eg. ZnO, CuS, FeS etc.

Example:

1. Quicklime contains 71.47% of calcium. How much calcium is present in a sample of quicklime which contains 16 g of oxygen?
   Solution:
   Quicklime contains 71.47% calcium
   So, percentage of Oxygen = (100-71.47) % = 28.53 %
   Let total weight be x
   Then 28.53% of x = 16 g
   Or, x= 56.08 g
   The weight of calcium = 56.08 – 16 = 40.08 g

References

1. thoughtco

2. britannica

3. wikipedia

4. study

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Part 1 of this article.

Laws of chemical combination #1: law of conservation of mass