Molar mass is measured in grams per mole; molecular mass is measured in atomic mass units.

• A. True
• B. False
• C. Only for elements
• D. They are the same quantity in all cases

Answer: (A)

The masses you’re talking about come from two related ideas: the mass per molecule and the mass per amount of substance. A molecule’s molecular mass is the sum of its atoms’ masses, expressed in atomic mass units (amu). The molar mass is the mass of one mole of those molecules, expressed in grams per mole (g/mol). Numerically these match for a given substance because 1 amu per particle corresponds to 1 g/mol when you scale up to a mole.

For example, water has a molecular mass of about 18.015 amu, and its molar mass is about 18.015 g/mol. One mole of water weighs about 18.015 grams. The same idea applies to elements as well, like iron with a molecular (atomic) mass of about 55.845 amu and a molar mass of about 55.845 g/mol. So the statement is true.