Question

How many water molecules does hydrated calcium sulphate contain?

• A. 5
• B. 10
• C. 7
• D. 2

Hint:

Remember the two main calcium sulphate hydration states:
- Gypsum = \(CaSO_4 \cdot 2H_2O\) (contains 2 water molecules).
- Plaster of Paris = \(CaSO_4 \cdot \frac{1}{2}H_2O\) (contains half a water molecule).

Answer 1

The Correct Option is D

Step 1: Identify hydrated calcium sulphate by its common name.
Hydrated calcium sulphate is the mineral gypsum, found naturally and used to make Plaster of Paris.
Step 2: Write its formula.
\[ CaSO_4 \cdot 2H_2O \] The number right before $H_2O$ tells us exactly how many water molecules are locked into each unit of the crystal.
Step 3: Compare with other common hydrates so there is no mix-up.
Blue vitriol carries 5 water molecules, washing soda carries 10, and green vitriol, iron sulphate, carries 7, none of these match calcium sulphate's own number.
Step 4: Conclude.
Gypsum, hydrated calcium sulphate, contains 2 water molecules. \[ \boxed{2} \]