Discussion
Chemical formula of plaster of Paris is
Answer: C
Plaster of Paris is \(\mathrm{CaSO_4 \cdot \tfrac{1}{2}H_2O}\), calcium sulphate hemihydrate.
It is made by heating gypsum to about 373 K (100 °C), which drives off three-quarters of its water of crystallisation:
\[ \begin{aligned} \mathrm{CaSO_4 \cdot 2H_2O} &\rightarrow \mathrm{CaSO_4 \cdot \tfrac{1}{2}H_2O} \\ &\quad + \mathrm{1\tfrac{1}{2}H_2O} \end{aligned} \]
When mixed with water it takes the water back and sets hard as gypsum again, which is why it is used for casts and moulds.
\(\mathrm{CaSO_4 \cdot 2H_2O}\) is gypsum itself, the anhydrous \(\mathrm{CaSO_4}\) is dead-burnt plaster formed by overheating, and the monohydrate is not the composition of plaster of Paris.
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