Experiment: Finding Formulae of Compounds (Edexcel IGCSE Science (Double Award))

Formulae of a simple compound by experiment

• 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 a 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 

Aim:

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

Diagram:

Heating a hydrated salt to remove the water of crystallisation

Method

1. Measure the mass of evaporating dish

2. Add a known mass of hydrated salt

3. Heat over a Bunsen burner, gently stirring, until the blue salt turns completely white, indicating that all the water has been lost

4. Record the mass of the evaporating dish and its contents

Practical tip:

• Avoid overheating the salt as it could decompose and give you a larger mass change

Results:

• Mass of the white anhydrous salt

  • Measure the mass of white anhydrous salt remaining

• Mass of water

  • Subtract the 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 (Step 1)	a / Mr  = y	b / Mr  =x
    Ratio (Step 2)	1 :	x

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