The Amount of Substance
Chemists use the moleThat chemical amount of a substance containing the same number of units as 12 g of carbon-12. so often to measure how much of a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. is present that it is convenient to have a name for the quantity which this unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. measures. In the International System this quantity is called the amount of substance and is given the symbol n. Here again a common English word has been given a very specific scientific meaning. Although amount might refer to volume or massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. in everyday speech, in chemistry it does not. When a chemist asks what amount of Br2 was added to a test tube, an answer like “0.0678 mol Br2” is expected. This would indicate that 0.0678 × 6.022 × 1023 or 4.08 × 1022, Br2 molecules had been added to the test tube.