Question

The number of water molecules in gypsum, dead burnt plaster and plaster of Paris, respectively are :

• A. 2, 0 and 1
• B. 0.5, 0 and 2
• C. 5, 0 and 0.5
• D. 2, 0 and 0.5

Hint:

Remember the sequence of heating gypsum: Gypsum (CaSO\(_4\)\(\cdot\)2H\(_2\)O) \(\xrightarrow{\sim 120^\circ\text{C}}\) Plaster of Paris (CaSO\(_4\)\(\cdot\)\(\frac{1}{2}\)H\(_2\)O) \(\xrightarrow{>120^\circ\text{C}}\) Dead Burnt Plaster (CaSO\(_4\)). This helps to recall the number of water molecules in each.

Answer 1

The Correct Option is D

To answer the question, we need to understand the composition of the given compounds: gypsum, dead burnt plaster, and plaster of Paris, and specifically, the number of water molecules they contain.

1. Gypsum:

   Gypsum is a mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO_4 \cdot 2H_2O. The formula indicates that each formula unit of gypsum contains two water molecules.

2. Dead Burnt Plaster:

   Dead burnt plaster is essentially anhydrous calcium sulfate. When gypsum is heated to about 200°C to 300°C, it loses its water content completely, forming anhydrous CaSO_4. Therefore, dead burnt plaster has zero water molecules in its structure.

3. Plaster of Paris:

   Plaster of Paris is a hemihydrate of calcium sulfate, which means its chemical formula is CaSO_4 \cdot \frac{1}{2}H_2O. This indicates that it contains half a molecule of water for each calcium sulfate unit.

Thus, the number of water molecules in gypsum, dead burnt plaster, and plaster of Paris are 2, 0, and 0.5, respectively.

Therefore, the correct answer is 2, 0, and 0.5.