When gypsum is heated to 393 K, it forms :

Solution

Chemistry ma’am · 4 steps

At 393 K gypsum gives up only three-quarters of its water and becomes plaster of Paris.

1. Start from gypsum. Gypsum is CaSO₄.2H₂O, with two water molecules for every CaSO₄ unit. Gentle heating drives off part of this water of crystallisation.

   2(CaSO₄.2H₂O) →(393 K) (CaSO₄)₂.H₂O + 3H₂O

2. Read the product. (CaSO₄)₂.H₂O has one water for every two CaSO₄ units, so it is written CaSO₄.0.5H₂O. This hemihydrate is plaster of Paris.
3. Above 393 K. Only on heating beyond 393 K is the remaining water lost. That gives anhydrous CaSO₄, called dead burnt plaster because it no longer sets with water.
4. So. At exactly 393 K the product is CaSO₄.0.5H₂O, option (a).

The idea to keep. Gypsum is the dihydrate, plaster of Paris is the hemihydrate at 393 K, and dead burnt plaster is anhydrous CaSO₄ above 393 K. Remember the water count at each temperature.

Answer from the NTA final answer key. Solution by PrepWiser.