JEE Main 2021 · 27 August · morning · Single correct · +4 / −1
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
Answer and solution
Answer (d) 2, 0 and 0.5
Solution
Chemistry ma’am · 4 steps
Follow the water as you heat gypsum: 2, then half, then none at all.
- Gypsum. Gypsum is the dihydrate, CaSO₄.2H₂O, so it carries 2 water molecules per CaSO₄.
- Plaster of Paris. Heating gypsum to about 393 K drives off part of the water and gives the hemihydrate, CaSO₄.1/2H₂O. That is 0.5 water molecule per CaSO₄, written as 2CaSO₄.H₂O when you clear the fraction.
CaSO₄.2H₂O →(393 K) CaSO₄.1/2H₂O + 3/2H₂O
- Dead burnt plaster. Heating above 393 K removes all the water and leaves anhydrous CaSO₄, with 0 water molecules. It is called dead burnt because it no longer sets with water.
- So. Gypsum, dead burnt plaster and plaster of Paris have 2, 0 and 0.5 water molecules, which is option (d).
The idea to keep. Gypsum (CaSO₄.2H₂O) loses water on heating: at 393 K it becomes plaster of Paris (CaSO₄.1/2H₂O), and stronger heating gives anhydrous dead burnt plaster.
Answer from the NTA final answer key. Solution by PrepWiser.