Moles from Mass

This formula tells you how many moles are present when you know the mass of a substance and its molar mass. It works because molar mass is the mass of 1 mole, so dividing the sample mass by the mass per mole tells you how many equal mole-sized portions are in the sample. The units also line up: grams divided by grams per mole leaves moles.

Use this when converting a measured mass in a chemistry problem into moles, such as before using a balanced equation or calculating concentrations. It is especially common in stoichiometry, solution chemistry, and lab calculations.

• Using the atomic mass of one element instead of the full compound molar mass
• Forgetting to divide by molar mass and instead multiplying by it
• Mixing up units, such as using mg or kg without converting to grams first