What is the chemical combination formula of Plaster of Paris?
- ACaSO₄·½H₂OCorrect
- BCaCO₃·2H₂O
- CCaSO₄·2H₂O
- DCaSO₃·2H₂O
Solution
Concept
Plaster of Paris is calcium sulphate hemihydrate. It is made by heating gypsum (calcium sulphate dihydrate).
Formula
CaSO₄·2H₂O → (heat ~150°C) → CaSO₄·½H₂O + 1½H₂O
Step-by-step solution
1. Gypsum is CaSO₄·2H₂O. 2. On heating to about 150°C, gypsum loses 1½ molecules of water. 3. The product is CaSO₄·½H₂O, which is Plaster of Paris.
Explanation
Plaster of Paris is made by partly heating gypsum. It keeps only half a molecule of water, so its formula is CaSO₄·½H₂O. When mixed with water, it sets back into gypsum.
Why the other options are wrong
B: CaCO₃ is calcium carbonate (limestone), not a sulphate. C: CaSO₄·2H₂O is gypsum, the raw material, not Plaster of Paris. D: CaSO₃ is calcium sulphite, which is the wrong compound.
Shortcut
Gypsum = 2 water. Plaster of Paris = half water (hemihydrate).
Common mistake
Confusing gypsum (CaSO₄·2H₂O) with Plaster of Paris (CaSO₄·½H₂O).
Correct answer: A) CaSO₄·½H₂O