Relative Formula Mass

Relative formula mass (Mr) is a concept used to determine the total mass of all atoms in a chemical formula. To calculate the Mr, we need to add up the relative atomic masses (Ar) of all the atoms present in the formula.

For example, we can figure out the mass of carbon dioxide (CO2) because we know it consists of one carbon atom (Ar = 12) and two oxygen atoms (Ar = 16)

Mr of CO2 = (1 x 12) + (2 x 16) = 12 + 32 = 44

  • Mr values don’t have any units because they are relative masses

Although ionic compounds do not exist as molecules, they can be calculated in the same way.


The formula for water is H2O

H2O has 2 hydrogen atoms and 1 oxygen atom. Hydrogen has an (Ar = 1) and oxygen has an (Ar = 16).

Mr of H2O = (2 × 1) + (1 × 16) = 2 + 16 = 18

The Conservation of Mass

The law of conservation of mass states that in a chemical reaction, the total mass of the reactants is equal to the total mass of the products. This means that matter is neither created nor destroyed during a chemical reaction.

We can use relative atomic masses and relative formula masses to demonstrate the conservation of mass in a chemical reaction.

Step 1: Balancing the Chemical Equation

The first step in demonstrating the conservation of mass is to write a balanced chemical equation. In this example, we’ll use the following reaction:

Zn + 2HCl → ZnCl2 + H2

Step 2: Finding the Relative Atomic Masses

Next, we’ll find the relative atomic masses of the elements involved in the reaction. The relative atomic mass of each element can be found on the periodic table.

  • Relative atomic mass of Hydrogen (H) = 1
  • Relative atomic mass of Zinc (Zn) = 65.4
  • Relative atomic mass of Chlorine (Cl) = 35.5

Step 3: Calculating the Relative Formula Masses

Next, we’ll use the relative atomic masses to find the relative formula masses of the compounds involved in the reaction. The relative formula mass of a compound is the sum of the relative atomic masses of the elements in its chemical formula.

  • Relative formula mass of Hydrochloric Acid (HCl) = 1 + 35.5 = 36.5
  • Relative formula mass of Zinc Chloride (ZnCl2) = 65.4 + (2 × 35.5) = 136.4
  • Relative formula mass of Hydrogen gas (H2) = 2 × 1 = 2

Step 4: Multiplying the Relative Formula Masses

Finally, we’ll use the balanced chemical equation to find the total relative formula masses of the reactants and products. We’ll multiply the relative formula mass of each substance by the coefficient in front of it in the chemical equation.

  • Relative formula mass of reactants: (Ar of Zn) + (2 × Mr of HCl) = 65.4 + (2 × 36.5) = 138.4
  • Relative formula mass of products: (Mr of ZnCl2) + (Mr of H2) = 136.4 + 2.0 = 138.4

Step 5: Comparing the Relative Masses

We can see that the relative masses of the reactants and products are the same, meaning that mass is conserved in the reaction.