Experiment: Finding Formulae of Compounds (Edexcel IGCSE Chemistry)

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

Aim:

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

Diagram:

Heating a hydrated salt to remove the water of crystallisation

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’