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## Laws of Chemical Combination The law of conservation of mass states that 'mass cam neither be created nor destroyed.' ### 1.4.2 Law of Definite Proportions: * French chemist, Joseph Proust performed experiments on two samples of cupric carbonate. * One of the samples was natural in origin and...

## Laws of Chemical Combination The law of conservation of mass states that 'mass cam neither be created nor destroyed.' ### 1.4.2 Law of Definite Proportions: * French chemist, Joseph Proust performed experiments on two samples of cupric carbonate. * One of the samples was natural in origin and the other was a synthetic one. * He found that the composition of elements present in it was the same for both the samples as shown below: | | % of copper | % of oxygen | % of carbon | |-----------|-------------|--------------|-------------| | Natural | 51.35 | 38.91 | 9.74 | | Synthetic | 51.35 | 38.91 | 9.74 | This led Joseph Proust to state the law of definite proportion as follows: * **A given compound always contains exactly the same proportion of elements by weight.** * Irrespective of the source, a given compound always contains same elements in the same proportion. * The validity of this law has been confirmed by various experiments. * This law is sometimes referred to as the Law of definite composition. ### 1.4.3 Law of multiple proportions: * This law was proposed by John Dalton in 1803. * It has been observed that two or more elements may form more than one compound. * The Law of multiple proportions summarizes many experiments on such compounds. * When two elements A and B form more than one compound, the masses of element B that combine with a given mass of A are always in the ratio of small whole numbers. * For example, * Hydrogen combines with oxygen to form two compounds, namely water and hydrogen peroxide. * **Hydrogen + Oxygen → Water** * 2 g + 16 g → 18 g * **Hydrogen + Oxygen → Hydrogen Peroxide** * 2 g + 32 g → 34 g Here, it is found that, the two masses of oxygen i.e. 16 g and 32 g which combine with a fixed mass of hydrogen (2g) are in the ratio of small whole numbers, i. e. 16:32 or 1:2. * **Nitrogen and oxygen combine to form two compounds, nitric oxide and nitrogen dioxide.** * **Nitrogen + Oxygen → Nitric Oxide** * 14 g + 16 g → 30 g * **Nitrogen + Oxygen → Nitrogen Dioxide** * 14 g + 32 g → 46 g Here, you find that the two masses of oxygen i.e. 16 g and 32 g when combine with a fixed mass of Nitrogen (14 g) are in the ratio of small whole numbers i.e. 16:32 or 1:2. * **(Similar examples such as CO and CO$_2$, (1:2 ratio), SO$_2$ and SO$_3$, (2:3 ratio), can be found)** ### 1.4.4 Gay Lussac Law of Gaseous Volume * This law was put forth by Gay Lussac in 1808. * The law states that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume, provided all gases are at the same temperature and pressure. **Illustration:** * Under the same conditions of temperature and pressure, 100 mL of hydrogen combines with 50 mL of oxygen to give 100 mL of water vapour. * **Hydrogen (g) + Oxygen (g) → Water(g)** * 100 mL (2 vol) + 50 mL (1 vol) → 100 mL (2 vol) * Thus, the volumes of hydrogen gas and oxygen gas which combine together ie. 100 ml. and 50 mL. producing two volumes of water vapour which amounts to 100 ml. bear a simple ratio of 2:1:2 * Under the same condition of temperature and pressure,