Answer:
I never thought of it as a tool, but I think you refer to Atomic Weight. The total atomic weight of the atoms in a molecule is its Mass in Moles.
Take a really simple example H2O. The atomic weight of Oxygen is 16, Hydrogen is 1, and so added, 1 + 1 + 16 = 18; the mass of 1 mole of water is 18g. Same for Benzene, C6H6; atomic weight of carbon is 12, so 6 x 12 = 72, add 6 x 1, or 6, and 72 + 6 = 78
Explanation: The sum of the atomic weights of the atoms in a molecule will give you the Molecular Weight, I.e. the Mass of 1 Mole of thst molecule.