What is the Definition of Formula Mass?

Formula Mass Definition

The formula mass of a molecule is the sum of the atomic weights of the atoms in the empirical formula of the compound.

Also Known As: formula weight

Formula Mass Units: Formula weight is given in atomic mass units (amu).

Formula Mass 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. Continue reading..