Most of the measurements and calculations in chemistry are concerned with quantities such as pressure, volume, mass, and energy. Every quantity includes both a number and a unit. The unit simultaneously identifies the kind of dimension and the magnitude of the reference quantity used as a basis for comparison. The number indicates how many of the reference units are contained in the quantity being measured. If we say that the mass of a sample is 20 grams, we mean that the mass is 20 times the mass of 1 gram, the unit of mass chosen for comparison. Although 20 grams has the dimension of mass, 20 is a pure dimensionless number, being the ratio of two masses, that of the sample and that of the reference, 1 gram.