Question

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 1.43g of copper oxide.
(c) In the third case, 1.45 g of copper when strongly heated in a current of air yielded 1.83 g of copper oxide.
Show that the given data illustrate the law of constant composition.

Answer 1

a) In the first experiment

2.19 g of CuO contained 1.75 g of Cu

100 g of CuO ------?

$\frac { 1.75 }{ 2.19 } \quad \times \quad 100\quad =\quad 79.91\quad g$

b) In the second experiment

1.43 g of CuO contained 1.14 g of Cu

100 g of CuO ------?

$\frac { 1.14 }{ 1.43 } \quad \times \quad 100\quad =\quad 79.72\quad g$

c) In the third experiment

1.83 g of CuO contained 1.45 g of Cu

100 g of CuO ------?

$\frac { 1.45 }{ 1.83 } \quad \times \quad 100\quad =\quad 79.23\quad g$

From the above three experiments, nearly same amount of copper oxide.

Hence, the above data illustrate the Law of constant composition.

Answer 2

Step 1: In the first experiment.

2.19 g of copper oxide contained 1.75 g of Cu.

100 g of copper oxide contained

Step 2: In the second experiment.

1.43 g of copper oxide contained 1.14 g of copper.

100 g of copper oxide contained

Step 3: In the third experiment.

1.83 g of copper oxide contained 1.46 g of copper

100 g of copper oxide contained

Thus the percentage of copper in copper oxide derived from all the three experiments is nearly the same. Hence, the above data illustrate the law of constant composition.

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