Define the law of multiple proportions. Explain it with two examples. How does this law point to the existence of atoms?
The correct answer and explanation is :
Law of Multiple Proportions (Definition):
The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.
Explanation:
This law was proposed by John Dalton in 1803. It supports the idea that chemical compounds are composed of atoms, which combine in specific ratios. According to the law, if element A combines with element B in more than one way to form different compounds, then the ratios of the masses of B that combine with a fixed mass of A will be small whole numbers.
Example 1: Carbon and Oxygen
Carbon forms two oxides:
- Carbon monoxide (CO): 12 g of carbon combines with 16 g of oxygen.
- Carbon dioxide (CO₂): 12 g of carbon combines with 32 g of oxygen.
Ratio of oxygen that combines with a fixed mass (12 g) of carbon = 16 : 32 = 1 : 2 (a simple whole-number ratio).
Example 2: Nitrogen and Oxygen
Nitrogen forms several oxides:
- Nitric oxide (NO): 14 g of nitrogen combines with 16 g of oxygen.
- Nitrogen dioxide (NO₂): 14 g of nitrogen combines with 32 g of oxygen.
Ratio of oxygen that combines with 14 g of nitrogen = 16 : 32 = 1 : 2.
Connection to Atomic Theory:
The law of multiple proportions provides strong evidence for the existence of atoms. If elements combined in arbitrary ratios, the masses would not follow simple whole-number patterns. The observed regularity suggests that elements consist of indivisible units (atoms) that combine in definite numbers (like 1, 2, or 3 atoms of one element with 1 atom of another). These consistent and simple mass ratios imply a discrete, atomic nature of matter, supporting Dalton’s atomic theory and reinforcing the concept that atoms are the fundamental building blocks of chemical compounds.