Question

2.8 g of calcium oxide (CaO) prepared by heating limestone was found to contain 0.8 g of oxygen. When one gram of oxygen was treated with calcium, 3.5 g of calcium oxide was obtained. Show that the results illustrate the law of definite proportions.

Answer 1

Knowing the basics of laws of chemical combinations; especially law of constant or definite proportion will help us solve the illustration.

Complete Solution :
Let us study the laws of chemical combinations:
- Law of reciprocal proportion-
The law of reciprocal proportion states that if two different elements combine separately with a fixed mass of any third element, the ratio in which they do so is the same to the ratio of their masses, when those two elements combine with each other.
This law is also known as the law of equivalent proportions or law of permanent ratios.

- Law of multiple proportion-
The law of multiple proportion states that if two elements react to form one or more compounds; then the ratios of the masses of the element with variable mass combining with the element of fixed mass can always be expressed as the ratio of small whole numbers.

- Law of constant proportion-
The law of constant proportion states that the compound forming from the combination of some elements will form in the fixed ratio of those elements irrespective of their source or method of preparation.
This law is also known as the law of definite proportion or law of constant proportion.

Illustration-
Given data:
Case I-
2.8 g of CaO contains 0.8 g of oxygen.
Thus, mass of calcium in CaO = 2.8 – 0.8 = 2 g
Ratio of mass of calcium to oxygen = $\dfrac{2}{0.8}=2.5$

Case II-
3.5 g of CaO contains 1 g of oxygen.
Thus, mass of calcium in CaO = 3.5 – 1 = 2.5 g
Ratio of mass of calcium to oxygen = $\dfrac{2.5}{1}=2.5$
Therefore, according to the above given definitions and the results we obtained we can say that the given data illustrates the law of definite proportions.

Note: Students may make a mistake of solving the above given question by considering the ratio of mass of CaO to oxygen. But this way of solving for proving the law is wrong though we get the proper answer. Do note, not to make such silly mistakes.