The study of matter in small quantities, such as atoms and molecules, is definitely difficult. In order to manage such small substances, chemists use large amounts of these small substances. The chemicals are measured using scales, graduated cylinders, or other macroscopic measuring devices. It is impossible to weigh one atom of carbon on a laboratory scale; however, it is possible to weigh a trillion, trillion carbon atoms. When enough atoms or molecules are put together, the mass becomes heavy enough that the laboratory scales can measure them relatively accurately. This is how atoms and molecules are counted in chemistry, by weighing and then converting.

The molar approach to studying atoms and molecules also helps to determine the actual formulas for compounds. Each compound has a unique and constant percent composition due to the constant mole ratio. Water, for example, always has two hydrogen atoms and one oxygen atom. In terms of percent composition, this means that water is 11.2% hydrogen and 88.8% oxygen by mass. It is important that you recognize the relationships between the mole ratio and percent composition by doing these various calculations.