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Describing Solutions

Author(s): David R. Caprette, PhD

Amedeo Avogadro and the Mole

The masses of two different molecular species may be very different, so expressing a concentration as weight-to-volume or weight-to-weight tells us nothing about the proportions of actual reactants in a solution. Thus, it is therefore more useful to know the number of molecules of solute per unit volume than the mass of solute per unit volume. It would be extremely awkward to express concentrations as numbers of molecules per liter, because for a typical solution that number is staggeringly large. A convenient and physically meaningful unit for describing a quantity of molecules is the mole. The number of molecules in one mole of a pure substance, namely 6.022 x 1023, is known as Avogadro's number, although Avogadro himself never calculated the number that bears his name.

Lorenzo Romano Amedeo Carlo Avogadro, conte di Quaregna e di Cerreto (1776 - 1856), was born in Turin, Italy. Avogadro's formal education was in law, and in fact, he had a successful legal career. In that era, many very basic scientific principles were as yet undiscovered, and it was possible for people such as Avogadro to pursue their interests in "natural philosophy" and actually make a lasting contribution to science.

In the early Nineteenth Century, John Dalton proposed that each atom of an element had a characteristic atomic weight, and that atoms were combined when chemical reactions took place. Around the same time, Gay-Lussac found that in chemical reactions involving gases, the ratios of volumes of the gases yielded small numbers that were integers not always equal to 1. If each substance was composed simply of a single atom, as Dalton had postulated, the ratios of volumes of reacting gases would have to be in unity.

In 1811, Avogadro made a distinction between atoms and molecules. For example, the "atoms" of nitrogen and oxygen are in reality "molecules, " each containing two atoms. Two molecules of hydrogen can combine with one molecule of oxygen to produce two molecules of water. Avogadro further suggested that at the same temperature and pressure, equal volumes of all gases contain the same number of molecules. This suggestion, which was borne out by later research, is known as Avogadro's principle.

Avogadro's number currently is based upon the definition of atomic mass, the atomic number for carbon, and the SI unit for mass. The atomic number for the common form of carbon is 12. A mole is defined as the number of atoms in 12 grams of pure carbon, which is 6.0221367 x 1023, with some uncertainty about the seventh decimal place. One mole is an incredibly large number-counting at a pace of one per second it would take 20 million billion years to count the atoms in one mole.