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: Understanding the Question:
The question asks for the number of water molecules of crystallization present in one formula unit of hydrated calcium sulphate.

Step 2: Key Formula or Approach:
Hydrated calcium sulphate is commonly known as Gypsum. We write its chemical formula to determine the hydration number.

Step 3: Detailed Explanation:

• Hydrated calcium sulphate is a naturally occurring mineral known as Gypsum.

• The chemical formula of Gypsum is:
\[ CaSO_4 \cdot 2H_2O \]

• This formula indicates that each formula unit of calcium sulphate (\(CaSO_4\)) is chemically bonded with exactly two molecules of water of crystallization.

• Let us review the other numbers:
- 5 molecules: Found in hydrated copper sulphate (Blue vitriol, \(CuSO_4 \cdot 5H_2O\)).
- 10 molecules: Found in hydrated sodium carbonate (Washing soda, \(Na_2CO_3 \cdot 10H_2O\)).
- 7 molecules: Found in hydrated iron sulphate (Green vitriol, \(FeSO_4 \cdot 7H_2O\)).

• Therefore, hydrated calcium sulphate contains 2 water molecules.


Step 4: Final Answer:
Hydrated calcium sulphate contains 2 water molecules, matching Option (D).