Summary Relative Formula Mass - Unit 1 - Principles and Applications of Science I

Relative Formula Mass

1. What is the relative formula mass of a compound?

is calculated by adding together the relative atomic mass values for all the atoms in
its formula.

2. What is the relative formula mass of:

a) MgCl2 = Mg(24), Cl (35.5 x 2)
24 + 71 = 95
b) C6H12O6 = C (12 x 6), H (1 x 12), O (16 x 6)
72 + 12 + 96 = 180

3. What can be said about the sum of the relative formula masses of the reactants and
products of a reaction?

The sum of the relative formula masses of the reactants is equal to the sum of
the relative formula masses of the products.

4. Why can you have relative atomic masses which are not whole numbers e.g. chlorine is
35.5?

This is because they contain 2 different isotopes of the element.



Calculate the Formulae of Simple Compounds:



Reacting masses Percentage composition

• Mass ÷ relative atomic mass • Percentage ÷ relative atomic mass

• Find the ratio • Find the ratio

• Use ratio to find empirical formula • Use ratio to find empirical formula


Calculate the empirical formula for a Scientists found a compound containing 75% carbon and
compound which contains 11.2g iron and 25% hydrogen. Calculate the empirical formula for the
4.8g oxygen. compound.

Relative atomic masses: Fe 56; O 16 Relative atomic masses: C 12; H 1

Fe O C H

Mass 11.2 4.8 % 75 25

RAM 56 16 RAM 12 1

Mass ÷ RAM 0.2 0.3 % ÷ RAM 6.25 25