Question
State the law of constant composition or Definite proportion. Explain with suitable examples.
Solution
Law of constant composition (Louis Proust, 1799) states that a pure chemical compound, regardless of its source, is always made up of the same elements, combined together in a fixed proportion by weight.
For example, carbon dioxide can be obtained by a number of methods:
(i) by burning coal or candle
(ii) by respiration
(iii) by heating limestone (CaCO3)
(iv) by the action of dilute hydrochloric acid on marble pieces.
It has been observed that each sample of carbon dioxide contains carbon and oxygen elements in the ratio of 3:8 by weight.
