Which one of the following is the number of water molecules that share with two formula unit CaS04 in plaster of Paris ?

Plaster of Paris is chemically known as calcium sulfate hemihydrate, represented by the formula CaSO4·1/2H2O [1]. This formula indicates that there is half a molecule of water of crystallization for every one formula unit of calcium sulfate. Since it is physically impossible to have half a water molecule, the structure is understood as two formula units of calcium sulfate (CaSO4) sharing a single molecule of water [3]. This relationship can also be expressed as (CaSO4)2·H2O [3]. Therefore, when considering two formula units of CaSO4, they share exactly one water molecule to maintain the hemihydrate ratio [1]. This chemical arrangement occurs when gypsum (CaSO4·2H2O) is heated to approximately 373 K, causing it to lose three-fourths of its water content to become the hemihydrate form [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Plaster of Paris > p. 33
2. [3] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Plaster of Paris > p. 32
3. [2] https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-Sulfate_-Hemihydrate