The molar mass is defined as...

• A. The mass of a single molecule
• B. The mass of one mole of a substance
• C. The volume of one mole of gas
• D. The number of particles in a mole

Answer: (B)

Molar mass is the mass of one mole of a substance. It tells you how much mass you have when you have Avogadro’s number of particles (6.022×10^23). In practice it’s measured in g/mol and for a given substance equals its relative molecular/formula mass. For example, water has a molar mass of about 18 g/mol because two hydrogens (1 each) plus one oxygen (16) add up to 18. The other ideas mix up what a quantity represents: the mass of a single molecule is the molecular mass (in atomic mass units), not per mole; the volume of one mole of gas relates to the space it occupies (molar volume), not its mass; and the number of particles in a mole is Avogadro’s number, a count, not mass.