Avogadro's Hypothesis and Avogadro Number
Avogadro's Hypothesis and Avogadro Number
Since, in chapter 7, Avogadro appears frequently, let me insert a brief note on Avogadro's Hypothesis and Avogadro Number. Amedeo Avogadro (1776-1856) is an Italian physicist-chemist. In 1811, he proposed a hypothesis on the number of molecules in a unit volume of any gas. Let me quote Perrin's exposition:
... equal volumes of different gases, under the same conditions of temperature and pressure, contain euqal numbers of molecules. The hypothesis may also be enumciated with advantage as follows:
When in the gaseous condition, equal numbers of molecules of any kind whatever, enclosed in equal volumes at the same temperature, exert the same pressure. (Perrin, Atoms, 18)
Today, this hypothesis is called "Avogadro's Law"; and it is supposed to hold for the ideal gas. Now, in physics and chemistry, "gramme molecule" (may be spelled as "gram molecule", and in this context, may be identified with "mole") is defined as the mass of any substance which occupy, in the gaseous state, the same volume of 32 grammes of oxigen (recall that oxigen molecule has two oxigen atoms), at the same temperature and pressure. For instance, one gramme molecule of hydrogen is about 2 grammes, and one gramme molecule of water is about 18 grammes. By Avogadro's Hypothesis, each gas should contain the same number of molecules in one gramme molecule. This number is Avogadro Number or Avogadro Constant.
Last modified Sept. 20, 2007. (c) Soshichi Uchii
