HTB ReStyle

2. Law of Definite Proportion

This law is used to describe the way elements combine during the formation of compounds. It states that a particular compound (regardless of its source) is always made of the same elements combined in the same proportion or ratio by mass. It is also known as the law of constant composition. It simply means that the mass fraction of an element in a compound is always constant. For instance, a pure sample of water would always be composed of 1/9 (0.1111) ratio by mass of hydrogen and 8/9 (0.8889) ratio by mass of oxygen. The mass ratio can also be expressed in percentage (i.e percentage by mass). In other words, a pure sample of water contains 11.11% and 88.89% by mass of hydrogen and oxygen elements respectively.

While law of conservation of mass enables us to write balanced chemical reaction equations, law of definite proportion enables us to write a definite chemical formula for a known compound.

Example 1: What mass of calcium would 20g of calcium carbonate yield, if 100g of the compound yielded 40g of calcium?

Solution:

Step i. Calcium carbonate is composed of calcium, carbon and oxygen. Remember, the mass ratio of each of the element would be constant.

Step ii. For calcium, the mass ratio would be mass of the element (calcium) divided by mass of the compound (calcium carbonate).

Step iii. The mass ratio of 40g of calcium in 100g of calcium carbonate would be 40g/100g=2/5.

Step iv: Therefore, in 20g of calcium carbonate, the mass of calcium would be 2/5 x 20g = 8g

Exercise:

A compound, Q, was found to be composed of carbon and oxygen alone. 84.2g of Q yieldded 71.4g of carbon, what would be the mass of oxygen in a 102g sample of Q?