Q1. [1] straightforward exam-ready
Which of the following is the correct chemical formula of Plaster of Paris?
(A) CaSO₄
(B) CaSO₄.½H₂O
(C) CaSO₄.H₂O
(D) CaSO₄.2H₂O
- A CaSO₄
- B CaSO₄.½H₂O
- C CaSO₄.H₂O
- D CaSO₄.2H₂O
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
The correct answer is (B) CaSO₄.½H₂O
Plaster of Paris is calcium sulphate hemihydrate, with the chemical formula CaSO₄.½H₂O.
Explanation
- CaSO₄ (option A) is anhydrous calcium sulphate (dead burnt plaster).
- CaSO₄.½H₂O (option B) is Plaster of Paris — the key point is the half water of crystallisation.
- CaSO₄.2H₂O (option D) is gypsum, the raw material from which Plaster of Paris is made by heating.
- Examiners commonly use all four options to test whether students can distinguish gypsum, POP, and anhydrous calcium sulphate — memorise the formula exactly.