Chemical formula of Plaster of Paris (POP) is:

Question

Accepted Answer

The correct option is (2):

Plaster of Paris (POP) is a white powdery chemical compound that is widely used in construction, sculpture, and medicine. Its chemical name is calcium sulfate hemihydrate. The term "hemihydrate" signifies that it contains half a molecule of water of crystallization for every molecule of calcium sulfate. This specific composition is crucial to its properties, particularly its ability to set and harden when mixed with water.

POP is manufactured by heating gypsum, which is calcium sulfate dihydrate ($\mathrm{CaSO_4 \cdot 2H_2O}$), to a temperature between $100^\circ\mathrm{C}$ and $190^\circ\mathrm{C}$ (or $160-170^\circ\mathrm{C}$ according to [3]). During this heating process, gypsum loses about $14.7%$ of its water content [3], specifically three-quarters of its water of crystallization. The chemical reaction can be represented as:

$\mathrm{CaSO_4 \cdot 2H_2O (gypsum) \xrightarrow{\Delta} CaSO_4 \cdot \frac{1}{2}H_2O (Plaster \ of \ Paris) + \frac{3}{2}H_2O (water \ vapor)}$

This reaction clearly shows that one molecule of gypsum loses $1.5$ molecules of water to form Plaster of Paris with $0.5$ molecules of water. Therefore, the chemical formula $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$ accurately represents calcium sulfate hemihydrate.

The incorrect option is (1):
$\mathrm{CaSO\_4 \cdot 5H\_2O}$ represents calcium sulfate pentahydrate. This is not the chemical formula for Plaster of Paris and implies an excessive amount of water molecules associated with calcium sulfate, which does not correspond to its known chemical structure or preparation.

The incorrect option is (3):
$\mathrm{CaSO\_4 \cdot 2H\_2O}$ is the chemical formula for gypsum, which is the raw material used to produce Plaster of Paris, not Plaster of Paris itself. Gypsum has two molecules of water of crystallization per molecule of calcium sulfate.

The incorrect option is (4):
$\mathrm{CaSO\_4 \cdot 10H\_2O}$ represents calcium sulfate decahydrate. This formula indicates ten molecules of water of crystallization, which is chemically inaccurate for Plaster of Paris and does not exist as a common stable hydrate of calcium sulfate.