Mole: Amount of Substance

James Richard Fromm


The word stoichiometry is a technical chemical term derived from the two Greek roots stoicheion, "element", and metron, "measure". Chemical stoichiometry is the area of chemistry which deals with the quantitative relationships between numbers of atoms involved in matter. Since very large numbers of atoms are needed to make up real objects which we can conveniently handle, the unit of amount of substance contains a large number of atoms. That unit of amount of substance is called the mole.

The mole is the SI unit for amount of substance. It is defined as a number - that number of atoms which exists in exactly twelve grams of the isotope of carbon of isotopic mass twelve. For historical reasons this number is known as the Avogadro number; its symbol is NA. Although the Avogadro number is very large, it can be measured with high precision; the currently accepted value is 6.0221367(36) x 10+23 entities/mole.

Moles can be used to measure the quantity of any substance. A mole of water is a small mouthful; a mole of elephants is about the size of the planet Saturn. For that reason we usually measure water, but not elephants, in moles while we usually measure elephants or eggs, but not water, in dozens. For chemical measurements, the mole is a very convenient unit while the dozen is not. A mole of sulfur has a mass of 32 g, a convenient mass to weigh out on a balance. An atom of sulfur would have a mass of about 5 x 10-23 g, and this mass is far too small to weigh out even on the most sensitive of balances. While the mole is the unit of amount of substance which chemists normally use, there are no measuring devices which directly measure moles. For this reason the amount of a substance is usually calculated from other measured quantities such as mass or volume.

Chemists use both intensive properties, which are characteristic of a substance and independent of the amount of the substance, and extensive properties, which are characteristic of a substance and directly proportional to the amount of the substance. Temperature, pressure, and color are intensive properties while mass and volume are extensive properties. It is often useful to give an extensive property in a form which is not directly proportional to amount of substance in other words, to convert an extensive property into an intensive property. This can be achieved by dividing the extensive property by amount of substance or by any other extensive property -- usually mass or volume. For example, the mass/volume ratio or density, d, is given by d = m/V.

The operation of division of one extensive property by another extensive property has the effect of transforming the extensive property into an intensive property. Density is an intensive property while both mass and volume are extensive properties. Intensive properties are of greater interest to chemists because they are much more characteristic of the nature of a particular substance.

Some extensive properties are transformed to intensive properties by dividing them by mass. Division by mass is usually indicated by preceding the name of the property with the word specific.


Example: the heat capacity of a substance is the amount of heat required to change its temperature, while the specific heat capacity or specific heat of a substance is the amount of heat required to change its temperature per gram or kilogram of the substance.


Chemists usually prefer to transform extensive properties to intensive properties by dividing them by amount of substance. This division by amount of substance is usually indicated by preceding the name of the property with the word molar. Examples of properties transformed in this way include the molar mass, the molar volume, and the molar heat capacity. Chemists usually concentrate on molar mass, because mass can be directly measured rather easily. Molar volume is taken up in a later section, and molar heat capacity is taken up in a different later section.


Copyright 1997 James R. Fromm