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

June 1, 2025 by rawan239

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

$ ext{CaSO}_4 cdot rac{1}{2} ext{ H}_2 ext{O}$

$ ext{CaSO}_4 cdot 2 ext{ H}_2 ext{O}$

$ ext{CaSO}_4 cdot 5 ext{ H}_2 ext{O}$

$ ext{CaSO}_4 cdot 4 ext{ H}_2 ext{O}$

This question was previously asked in

UPSC NDA-2 – 2021

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The correct option is A. The chemical formula of Plaster of Paris is Calcium Sulfate hemihydrate, which is $\text{CaSO}_4 \cdot \frac{1}{2} \text{ H}_2\text{O}$.

Plaster of Paris is produced by heating Gypsum ($\text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$) to about 150°C (302°F). This heating process removes three-quarters of the water of crystallization, leaving the hemihydrate form. When mixed with water, Plaster of Paris rehydrates and sets into a hard mass, which is chemically Gypsum.

Gypsum ($\text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$) is Calcium Sulfate dihydrate. It is a naturally occurring mineral. The setting of Plaster of Paris is an exothermic reaction: $\text{CaSO}_4 \cdot \frac{1}{2} \text{ H}_2\text{O} + 1\frac{1}{2} \text{ H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$.

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