Which quantity is defined as the mass of one mole of a substance in grams per mole?

• A. Atomic number
• B. Avogadro's number
• C. Mass number
• D. Molar mass

Answer: (D)

Molar mass is the mass of one mole of a substance, expressed in grams per mole. It connects the tiny masses you see on the atomic scale (in atomic mass units) to practical lab masses in grams. For a pure element, its molar mass equals the element’s atomic weight in g/mol as listed on the periodic table. For a compound, you add up the atomic masses of all atoms in its formula (taking into account the subscripts); the total in amu is numerically equal to the molar mass in g/mol. For example, water has a molar mass of about 18.02 g/mol because it contains two hydrogens (approx. 1.01 each) and one oxygen (about 16.00), summing to 18.02 amu, which translates directly to 18.02 g per mole. Because of this, one mole of any substance weighs its molar mass in grams. The other quantities don’t describe mass per mole: atomic number counts protons, Avogadro’s number is the number of particles in a mole, and mass number is protons plus neutrons in a single nucleus.