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
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.
Practical Science 