Edexcel IGCSE Chemistry

Revision Notes

1.5.6 Experiment: Finding Formulae of Compounds

Formulae of Simple Compounds by Experiment

Making Careful Quantitative Measurements

  • The formulae of simple compounds can be found by careful experimentation and accurate measurements of mass changes
  • The principle is to use mass measurements before and after reaction and then convert masses into moles
  • Using the moles of reactants and products it is possible to deduce molar ratios and hence an empirical formula
  • Experiments which are easier to do using this process involve gases being lost or gained
  • In this example a hydrated salt is heated to drive off the water as water vapour

The Formula of a Hydrated Salt

To determine the formula of hydrated copper sulfate, CuSO4. xH2O


Water of Crystallisation, downloadable IGCSE & GCSE Chemistry revision notes

Heating a hydrated salt to remove the water of crystallisation


  • Measure mass of evaporating dish
  • Add a known mass of hydrated salt
  • Heat over a Bunsen burner, gently stirring, until the blue salt turns completely white, indicating that all the water has been lost
  • Record the mass of the evaporating dish and contents

Practical tip:
Avoid overheating the salt as it could decompose and give you a larger mass change

Mass of the white anhydrous salt:
Measure mass of white anhydrous salt remaining

Mass of water:
Subtract mass of the white anhydrous salt remaining from the mass of known hydrated salt

Step 1 – Divide the mass of the copper sulfate and the water by their respective molar masses

Step 2 – Simplify the ratio of water to copper sulfate

              anhydrous salt      water

Mass               a                       b

Moles              a / Mr               b / Mr

     = y                    = x

Ratio                1          :           x

Step 3 – Represent the ratio in the form ‘salt.xH2O’

Exam Tip

It is unlikely that you will get a whole number for the number of moles of water in the ratio, so you will need to round up or down to the nearest whole number.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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