By David Chandler
Updated Aug 30, 2022
In everyday life we often use words like “dozen” or “pair” to describe quantities. In chemistry, a similar concept exists: the mole (mol). This unit represents 6.022 × 1023 entities, a value derived from the exact number of atoms in 12 g of the isotope carbon‑12. Using the mole, the molecular weight of a compound can serve as a direct conversion factor between moles and grams.
Write the chemical formula followed by the number of moles. The formula lists the atoms that make up one molecule—for example, H₂O for water. Place the mole quantity before the formula and omit the unit when the value is one. Thus 2 mol H₂O is written as 2 H₂O, while 1.8 mol H₂O is written as 1.8 mol H₂O.
Calculate the molecular weight by adding the atomic weights of all atoms in the formula. Atomic weights are given on the periodic table. For water, 2 × 1.008 (⁴H) + 16.00 (⁸O) = 18.02 g mol⁻¹.
Multiply the molecular weight by the number of moles to obtain the mass. For example, 1 mol H₂O × 18.02 g mol⁻¹ = 18.02 g. 2 mol H₂O × 18.02 g mol⁻¹ = 36.04 g. A fractional amount, such as 1.8 mol H₂O, yields 32.44 g.
