Formula Mass Definition and Example Calculation

Formula mass is the sum of the atomic weights of atoms in a molecule.
Formula mass is the sum of the atomic weights of atoms in a molecule. Chad Baker/ Digital Vision/ Getty Images

The formula mass of a molecule (also known as formula weight) is the sum of the atomic weights of the atoms in the empirical formula of the compound. Formula weight is given in atomic mass units (amu).

Example and Calculation

The molecular formula for glucose is C6H12O6, so the empirical formula is CH2O.

The formula mass of glucose is (12)+2(1)+16 = 30 amu.

Relative Formula Mass Definition

A related term you should know is relative formula mass (relative formula weight).

This simply means the calculation is performed using relative atomic weight values for the elements, which are based on the natural isotopic ratio of elements found in the Earth's atmosphere and crust. Because relative atomic weight is a unitless value, relative formula mass technically does not have any units. However, grams are often used. When the relative formula mass is given in grams, then it is for 1 mole of a substance. The symbol for relative formula mass is Mr and it is calculated by adding together the Ar values of all the atoms in the formula of a compound.

Relative Formula Mass Example Calculations

Find the relative formula mass of carbon monoxide, CO.

The relative atomic mass of carbon is 12 and of oxygen is 16, so the relative formula mass is:

12 + 16 = 28

To find the relative formula mass of sodium oxide, Na2O, you multiply the relative atomic mass of sodium times its subscript and add the value to the relative atomic mass of oxygen:

(23 x 2) + 16 = 62

One mole of sodium oxide has a relative formula mass of 62 grams.

Gram Formula Mass

Gram formula mass is the amount of a compound with the same mass in grams as the formula mass in amu. It is the sum of atomic masses of all atoms in a formula, regardless of whether or not the compound is molecular.

Gram formula mass is calculated as:

gram formula mass = mass solute / formula mass of solute

You'll usually be asked to give the gram formula mass for 1 mole of a substance.

Example

Find the gram formula mass of 1 moles of KAl(SO4)2 · 12H2O.

Remember, multiply the values of atomic mass units of atoms times their subscripts. Coefficients are multiplied by everything that follows. For this example, that means the are 2 sulfate anions based on the subscript and there are 12 water molecules based on the coefficient.

1 K = 39
1 Al = 27
2(SO4) = 2(32 + 16 x 4) = 192
12 H2O = 12(2 + 16) = 216

So, the gram formula mass is 474 g.