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Experiment: Finding Formulae of Compounds (Edexcel IGCSE Chemistry: Double Science)
Revision Note
Formula 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
- Measure the 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 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
= yb / Mr
=xRatio (Step 2) 1 : x - Step 3 – Represent the ratio in the form ‘salt.xH2O’
Examiner 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.
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