Copper oxide was prepared by the following methods :
(a) In one case, 1.75 g of the metal were dissolved in nitric acid and igniting the residual copper nitrate yielded 2.19 g of copper oxide.
(b) In the second case, 1.14 g of metal dissolved in nitric acid were precipitated as copper hydroxide by adding caustic alkali solution. The precipitated copper hydroxide after washing, drying and heating yielded 1.43 g of copper oxide.
(c) In the third case, 1.45 g of copper when strongely heated in a current of air yielded 1.83 g of copper oxide. Show that the given data illustrate the law of constant composition.

Explanation

Step 1. In the first experiment.
2.19 g of copper oxide contained 1.75 g of Cu.
`therefore" 100 g of copper oxide contained Cu"=(1.75)/(2.19)xx100=79.91g`, i.e., `%" of Cu"= 79.91`
Step 2. In the second experiment.
1.43 g of copper oxide contained 1.14 g of copper
`therefore" 100 g of copper oxide contained Cu"=(1.14)/(1.43)xx100=79.72g`, i.e., `%" of Cu"=79.72`
Step 3. In the third experiment.
1.83 g of copper oxide contained 1.46 g of copper
`therefore" 100 g of copper oxide contained Cu"=(1.46)/(1.83)xx100=79.78g`, i.e., `%" of Cu"=79.78`
Thus, the percentage of copper oxide derived from all the three exkperiments is nearly the same. Hence the above data illustrate the law of constant composition.