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Avogadro law

Laws of Chemical Combination



The combination of elements to form compounds depends on some basic laws.There are 5 laws of chemical combination.

Law of conservation of mass

The law was stated as matter can neither be created nor destroyed. This law was stated by Antoine Lavoisier in 1789. He performed various careful experimental studies for combustion reactions in different ways for reaching the above conclusion. This law later formed the basis for several developments in chemistry. This was the result of the exact measurement of masses of reactants and products and carefully planned experiments performed by Lavoisier.

Law of Definite Proportions

This law was given by Joseph Proust, a French chemist. It states that a given compound always contains exactly the same proportion of elements by weight. He worked with two samples of cupric carbonate, one of which was of natural origin and the other was synthetic one. He found that the composition of elements present in it was the same for both samples. Thus, irrespective of the source used, a given compound always contains the same amount of elements in the same proportion. The authenticity of this law has been confirmed with the help of various experiments. The law can also be referred to as Law of definite composition.

Law of Multiple Proportions

This law was stated by Dalton in 1803. This law states that, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. For example, hydrogen unites with oxygen to form two compounds. They are water and hydrogen peroxide.

Here the masses of oxygen (i.e. 16 g and 32 g)which unite with a fixed mass of hydrogen form a simple ratio of 16:32 i.e. 1:2.

Law of Gaseous Volumes

This law was stated by Gay Lussac in 1808. His observation was 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. Thus 100 ml of oxygen combine with 50 ml oxygen to give 100 ml of water vapor.

Hence, the volumes of hydrogen and oxygen which unite together to bear a simple ratio of 2:1. The finding of Gay-Lussac for integer ratio in volume relationship can actually be stated as the law of definite proportions by volume. The law of definite proportions was stated with respect to mass. Gay-Lussac’s law was explained in detail by the work of Avogadro in 1811.

Avogadro Law

In 1811, Avogadro proposed that equal volumes of gases at the same temperature and pressure should contain an equal number of molecules. He made a distinction between atoms and molecules which can easily be understood in present times. If the reaction of hydrogen and oxygen to produce water is taken as an example, it can be seen that two volumes of hydrogen unite with one volume of oxygen to form two volumes of water, without leaving any unreacted oxygen. Avogadro’s experiment was published in the French Journal named, de Physique. Though the experiments were correct it didn’t gain much fame and support. After about 50 years, in 1860 the first international conference on chemistry was held in Karlsruhe, Germany to resolve various ideas. In the meeting, Stanislao Cannizaro presented a sketch of a course of chemical philosophy that emphasized the importance of Avogadro’s work.

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