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Some Basic Concepts of Chemistry 15 1.5.2 Law of Definite Proportions are produced in a chemical This law was given by, a reaction they do so in a French chemist, Joseph simple ratio by volume, provided all gases are at Proust. He stated that a given compound always contains the same temperature and exactly the same proportion of pressure. elements by weight. Thus,100mLofhydrogen Joseph Louis Proust worked with two Joseph Proust combine with 50 mL of Gay Lussac samples of cupric carbonate (1754–1826) oxygen to give 100 mL of water vapour. — one of which was of natural origin and the other was synthetic. He found Hydrogen + Oxygen → Water that the composition of elements present in it 100 mL 50 mL 100 mL wassameforboththesamplesasshownbelow: Thus, the volumes of hydrogen and % of % of % of oxygen which combine (i.e., 100 mL and copper carbon oxygen 50 mL) bear a simple ratio of 2:1. Natural Sample 51.35 9.74 38.91 Gay Lussac’s discovery of integer ratio Synthetic Sample 51.35 9.74 38.91 in volume relationship is actually the law of definite proportions by volume. The law of Thus, he concluded that irrespective of the definite proportions, stated earlier, was with source, a given compound always contains respect to mass. The Gay Lussac’s law was same elements combined together in the same explained properly by the work of Avogadro proportion by mass. The validity of this law in 1811. has been confirmed by various experiments. It is sometimes also referred to as Law of 1.5.5 A vogadro’s Law Definite Composition. In 1811, Avogadro proposed that equal 1.5.3 L aw of Multiple Proportions volumes of all gases at the same temperature and pressure should contain equal number This law was proposed by Dalton in 1803. of molecules. Avogadro made a distinction According to this law, if two elements can combine to form more than one compound, between atoms and molecules which is quite understandable in present times. If the masses of one element that combine with we consider again the reaction of hydrogen a fixed mass of the other element, are in the and oxygen to produce water, we see that ratio of small whole numbers. two volumes of hydrogen combine with one For example, hydrogen combines with volume of oxygen to give two volumes of water oxygen to form two compounds, namely, water without leaving any unreacted oxygen. and hydrogen peroxide. Note that in the Fig. 1.9 (Page 16) each Hydrogen + Oxygen → Water box contains equal number of 2g 16g 18g molecules. In fact, Avogadro Hydrogen + Oxygen → Hydrogen Peroxide could explain the above result 2g 32g 34g by considering the molecules to be polyatomic. If hydrogen Here, the masses of oxygen (i.e., 16 g and 32 g)and oxygen were considered which combine with a fixed mass of hydrogen (2g) bear a simple ratio, i.e., 16:32 or 1: 2. as diatomic as recognised now, then the above results Lorenzo Romano 1.5.4 Gay Lussac’s Law of Gaseous are easily understandable. Amedeo Carlo V olumes However, Dalton and others Avogadro di Quareqa edi This law was given by Gay Lussac in 1808. believed at that time that Carreto He observed that when gases combine or atoms of the same kind (1776–1856) Reprint 2026-27