Laws of Chemical Combination - Law of Constant Proportions (Law of Definite Proportions)

French scientist J. L. Proust stated the law of constant proportion in 1794, explaining that the elements in a chemical compound always combine in a fixed proportion by mass. This means that no matter the source of a compound, its composition remains the same.

For example:

• In water, hydrogen and oxygen are always present in the mass ratio of 1:8. This implies that 1 g of hydrogen and 8 g of oxygen combine to form 9 g of water.
• Carbon and oxygen combine in carbon dioxide at a mass ratio of 3:8. For instance, in 44 g of carbon dioxide, there are 12 g of carbon and 32 g of oxygen.

Proust

Antoine Lavoisier (1743–94) was a French scientist known as the father of modern chemistry. He made significant contributions to chemistry, biology, and economics. He introduced the names "oxygen" and "hydrogen" for elements.

• Demonstrated that matter combines with oxygen during combustion.
• He was the first to use precise weighing methods in chemical experiments to measure reactants and products.
• Discovered that water is composed of hydrogen and oxygen.
• Formulated the law of conservation of mass in chemical reactions.
• Developed a systematic naming system for compounds, such as sulphuric acid and copper sulphate.
• Wrote the first book on modern chemistry, Elementary Treatise on Chemistry, in 1789.
• Conducted studies on elements like oxygen, hydrogen, nitrogen, phosphorus, mercury, zinc, and sulphur.

Lavoisier

The law of constant proportion can be verified using compounds prepared by different methods.

• Two samples of copper oxide (CuO) were obtained: one from the decomposition of copper carbonate (CuCO₃) and the other from copper nitrate (Cu(NO₃)₂).
• Each sample contained 8 g of copper oxide, which was reacted with hydrogen gas, producing 6.4 g of copper and 1.8 g of water.
• Water (H₂O) has a fixed proportion of hydrogen and oxygen by mass, 1:8. From this, the 1.8 g of water contained 1.6 g of oxygen.
• This oxygen originated from the copper oxide, meaning 8 g of copper oxide contained 6.4 g of copper and 1.6 g of oxygen.
• The proportion by mass of copper to oxygen in copper oxide was the same for both samples, 6.4:1.6 or 4:1, confirming the law of constant proportion.

Expected proportion from molecular formula CuO:  

From the known atomic masses of Cu and O,

Atomic mass of Cu = 63.5, atomic mass of O = 16.

Proportion by mass:

Cu:O = 63.5:16 = 3.968:1   or   approximately   4:1.

The experimental value of proportion by mass matched with the expected proportion calculated from the molecular formula. Thus, the law of constant proportion is verified.