ACS Organic Chemistry Practice Test

The main idea is that molar mass is the mass of one mole of a substance, expressed in grams per mole. This ties microscopic masses to a macroscopic amount of material: you can take the number of moles and multiply by this mass to get the total grams you have. For a pure element, the molar mass in g/mol equals the atomic mass in atomic mass units (amu). For a compound, you add up the atomic masses of all atoms in the formula, accounting for how many of each atom are present (for example, water has a molar mass about 18.015 g/mol: 2 × 1.008 for hydrogen plus 16.00 for oxygen). It’s not the mass of a single atom in grams, and it isn’t the mass of solvent per mole or a volume per mole—that would be molar volume. Therefore, the correct definition is the mass per mole, expressed as g/mol.