Mass

What is atomic mass?

Atomic mass is the average of the isotopic masses of an element based on their percentage of abundance in nature. This number is dimensionless and based on the carbon-12 atom, i.e. carbon has an atomic mass of 12 $u$ by definition.

What is molecular mass?

Also known as molecular weight, molecular mass is the weight of one molecule of a substance and can be calculated by summing the atomic mass ($u$) of the atoms in the molecule.

For example, the molecular mass of water, H$_{2}$O, is equal to the atomic mass of two hydrogens (1.007 $u$ per hydrogen) plus the atomic mass of oxygen (15.999 $u$) or 18.006 $u$.

What is molar mass?

The mass, in grams, of one mole of an element of atoms, molecules or particles (grams/mol).

What is a mole?

6.022 x 10$^{23}$ atoms, molecules or any given particle. This number is also known as Avogadro's number.

Therefore, because atomic mass ($u$) is based on carbon-12 and molar mass (grams/mol) is also based on carbon-12, 1 $u$ = 1 gram/mol.

From our example above, we calculated that H$_{2}$O is equal to a molecular mass of 18.006 $u$, with the conversion explained above, 18.006 $u$ = 18.006 grams/mol H$_{2}$O.