Objective
To determine the number of moles of water of crystallisation in hydrated copper(II) sulfate by heating it to constant mass.
Background
Hydrated copper(II) sulfate has the formula: CuSO₄·xH₂O.
When heated, it loses water and becomes anhydrous copper(II) sulfate (white powder):
CuSO₄·xH₂O (s) → CuSO₄ (s) + xH₂O (g). By measuring the mass before and after heating, we can calculate the value of x.
Apparatus
– Crucible
– Tripod and pipe-clay triangle
– Bunsen burner
– Heatproof mat
– Tongs
– Balance (2 d.p.)
– Hydrated copper(II) sulfate
Safety
Safety
⚠️ Hydrated copper(II) sulfate is an irritant — wear safety goggles, handle the solid with a spatula and avoid skin contact. Heating crystals to drive off water requires a Bunsen flame and a porcelain dish; the dish becomes very hot and must be handled with tongs. Stop heating once colour change to white anhydrous form is complete to avoid decomposition. Anhydrous copper sulfate reacts exothermically with water, so allow it to cool before adding water if the rehydration step is performed. Dispose of solid waste into the copper-salts waste container.
Method
1. Weigh the empty crucible. Record the mass.
2. Add about 2-3 g of hydrated copper(II) sulfate. Weigh and record the total mass.
3. Heat the crucible gently for 2 minutes or until the powder turns white.
4. Continue heating if you see any blue crystals remaining and use a glass rod to gently crush the crystals into a powder.
5. Allow to cool, then weigh the crucible and contents.
6. Repeat heating and weighing until a constant mass is achieved.
Results Table
| Measurement | Mass (g) |
| Mass of empty crucible + lid | |
| Mass of crucible + lid + hydrated CuSO₄ | |
| Mass of crucible + lid + anhydrous CuSO₄ | |
| Mass of water lost |
Calculations
- Mass of hydrated CuSO₄ used =
2. Mass of water lost =
3. Moles of anhydrous CuSO₄ = mass ÷ 159.61 g/mol
4. Moles of water lost = mass ÷ 18.02 g/mol
5. Mole ratio of H₂O to CuSO₄ =
6. Value of x (round to nearest whole number) =
1. Why is it important to heat to constant mass?
2. What would happen if the sample was not fully dehydrated?
3. Suggest a reason for any discrepancy between your result and the theoretical value of x = 5.
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