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Q94 (NDA-II/2021) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Applied everyday chemistry

Which one of the following is the chemical formula of Plaster of Paris ?

Result
Your answer: —  Â·  Correct: A
Explanation

Plaster of Paris is chemically known as calcium sulfate hemihydrate, represented by the formula CaSO4·1/2H2O [1]. It is prepared by heating gypsum (calcium sulfate dihydrate, CaSO4·2H2O) to approximately 373 K (100°C), causing it to lose three-fourths of its water of crystallization. The notation '1/2 H2O' indicates that two formula units of CaSO4 share a single molecule of water within the crystal lattice [1]. When mixed with water, Plaster of Paris undergoes a rehydration reaction to reform gypsum, creating a hard solid mass used for supporting fractured bones and in construction [1]. While the formula is often written as CaSO4·0.5H2O, it can also be represented as (CaSO4)2·H2O to reflect the sharing of water molecules. Option 1 correctly represents the hemihydrate formula despite the typographical spacing.

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Plaster of Paris > p. 33
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