Relative formula mass

Atoms have very little mass so instead of using their actual masses in kilograms, their relative atomic masses are used. The relative atomic mass of an element, symbol Ar, is defined as the relative mass of its atoms compared to the mass of a particular carbon atom (carbon-12). The Ar values for elements are often stated in the periodic table. Note, as Ar is a measure of the relative masses of atoms, it has no units.

The relative atomic masses of elements are proportional measures. For example, the Ar for carbon is 12, and the Ar for magnesium is 24. This means that magnesium atoms are twice the mass of carbon atoms.

Calculating relative formula mass

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The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Relative formula mass has the symbol, Mr. To calculate Mr for a substance:

  1. work out how many atoms of each element are in the chemical formula
  2. add together the Ar values for all the atoms of each element

For example, the formula for carbon dioxide is CO2. It has one carbon atom (Ar = 12) and two oxygen atoms (Ar = 16):

Mr of CO2 = 12 + 16 + 16 = 44

It could also be calculated this way:

Nr of CO2 = (1 × 12) + (2 × 16) = 12 + 32 = 44

Like Ar values, Mr values also have no units.

Relative formula masses of ionic compounds

Ionic compounds such as sodium chloride do not exist as molecules. However, their relative formula masses are calculated in the same way. The formulae used are their empirical formulae.

Question

Calculate the relative formula mass, Mr, of calcium hydroxide, Ca(OH)2.

(Ar of Ca = 40, Ar of O = 16, Ar of H = 1)

Mr = 40 + (2 × 16) + (2 × 1)

= 40 + 32 + 2

= 74

It could also be calculated this way:

Mr = 40 + 2 × (16 + 1)

= 40 + 34 = 74