When given a compound, adding up all of the of all of the individual atoms within one molecule of a compound will determine the molecular mass.

Answer is: atomic masses.

For example, calculate molecular mass of one mole of ethanol (C₂H₅OH). Molecule of ethanol has two carbon atoms, six hydrogen atoms and one oxygen atom:

M(C₂H₅OH) = 2Ar(C) + 6Ar(H) + Ar(O) · g/mol.

M(C₂H₅OH) = 2·12 + 6·1 + 16 · g/mol.

M(C₂H₅OH) = 46 g/mol; molar mass.

Atomic mass is approximately equivalent to the number of protons and neutrons in the atom.