AVOGADRO’S HYPOTHESIS - Theoretical Importance AVOGADRO’S HYPOTHESIS

AVOGADRO’S HYPOTHESIS

If we consider the case of elementary gases according to Gay Lussac’s law of combining volumes they combine together in simple ratio by volume. According to Dalton’s atomic theory, they combine together in simple ratios by number of atoms. Therefore the scientist Berzelius thought that there was some sort of relation between the volume of the gas and the number of atoms. He thought that the same number of atoms was present in the same volume. He proposed his hypothesis as follows.

Equal volumes of all gases under the same conditions of temperature and pressure have the same number of atoms. If we apply this hypothesis to the combination of hydrogen and chlorine we find that it is against Dalton’s atomic theory. One volume of hydrogen combines with one volume of chlorine to form two volumes of hydrogen chloride. Applying Berzelius's hypothesis let n be the number of atoms present in one volume of the gas n atoms of hydrogen combine with n atoms of chlorine to form 2n atoms of hydrogen combined with n atoms of chlorine to form 2n atoms of hydrogen chloride. Since n is the common factor one atom of hydrogen combines with one atom of chlorine to form 2 atoms of hydrogen chloride. In other words, one atom of hydrogen combines with one atom of chlorine to form 2 atoms of hydrogen chloride containing ½ atom of hydrogen and ½ atom of chlorine. This is against Dalton’s atomic theory because atoms cannot be divided. So there was a lot of confusion in the chemical world, some supporting Dalton others supporting Berzelius. Afterwards, Avogadro came to the field and made for the first time a distinction between atoms and molecules. He said that if the word molecule was substituted for the word atom in the Berzelius hypothesis then the hypothesis which is known as Avogadro’s hypothesis states that equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules.

Let us apply this hypothesis to the combination of hydrogen and chlorine. 1 volume of hydrogen combines with 1 volume of chlorine to form 2 volumes of hydrogen chloride. Applying Avogadro’s hypothesis let n be the number of molecules present in one volume of the gas. The n molecules of hydrogen combine with n molecules of chlorine to form 2n molecules of hydrogen chloride. Since n is the common factor, one molecule of hydrogen combines with 1 molecule of chlorine to form 2 molecules of hydrogen chloride or 1 molecule of hydrogen chloride contains ½ molecule of hydrogen and ½ molecule of chlorine. This is not against Dalton’s atomic theory because molecules unlike atoms can be divided.

Theoretical Importance AVOGADRO’S HYPOTHESIS

o  It gives a theoretical explanation of Gay Lussac’s law of combining volumes.

o  It proves that the molecules of elementary gases like hydrogen, nitrogen, oxygen and chlorine are all diatomic.

o  It proves that the molecular weight of a gas is twice its vapor density.

o  The gram molecular volume of all gases and vapors is the same and is equal to 22.4 liters at N.T.P.

o  It gives a method of arriving at the molecular formula of a gaseous compound from its volumetric composition.