Which equation correctly relates mass, moles, and molar mass?

• A. mass = molar mass ÷ moles
• B. mass = moles × molar mass
• C. mass = molar mass
• D. mass = moles + molar mass

Answer: (B)

Molar mass tells you the mass of one mole of a substance, so to get the total mass you multiply the number of moles by the mass per mole. If you have, say, 2 moles and the molar mass is 44 g/mol, the mass is 2 × 44 = 88 g. The units work out as grams because moles times g/mol gives g.

The other ideas don’t fit because dividing the molar mass by the number of moles would give a quantity with the wrong units (not a mass), saying mass equals the molar mass ignores how many moles you have, and adding the two quantities mixes incompatible quantities.