Water of Crystallisation

Question 1

Explain the water of crystallisation practical to work out the number of moles of water.

Answer 1

1~ Weigh mass of empty boiling tube using a balance with 0.01g resolution.
2~ Place a few spatulas of hydrated CuSO₄ in tube.
3~ Weigh mass of the boiling tube with hydrated copper sulfate.
4~ Calculate mass of hydrated CuSO₄ added (subtracting tube w/ CuSO₄ and empty tube).
5~ Heat CuSO₄ over a roaring Bunsen burner flame.

6~ Weigh boiling tube every couple of mins until the mass stops decreasing.
7~ Calculate mass of anhydrous CuSO₄ (subtracting tube w/ anhydrous CuSO₄ from empty tube)
Difference between mass of hydrous and anhydrous CuSO₄ is mass of water.
8~Then calculate the number of moles of water.

Question 2

Why do you heat the over a roaring bunsen burner flame?

Answer 2

This drives off the water of crystallisation as steam.

Question 3

Why do you weigh the tube every couple of minutes until the mass stops decreasing?

Answer 3

Ensures all the water is driven off and left with anhydrous copper sulfate.

Question 4

How do you carry out a water of crystallisation calculation?

Answer 4

SAME AS EMPIRICAL FORMULA, except we’re dealing with molecules instead of elements.

You have mass of hydrous and anhydrous.

1~ Calculate mass of water by subtracting the two.
2~ Calculate no. of moles of anhydrous CuSO₄ and water.

3~ Find the ratio between the moles by dividing by the smaller moles.
This gives you the number of moles of water in the hydrous compound copper sulfate.