Discussion
The molecular formula of Plaster of Paris is :
Answer: B
The answer the key accepts is \(\mathrm{2CaSO_{4} \cdot \tfrac{1}{2}H_{2}O(s)}\).
Plaster of Paris is calcium sulphate hemihydrate, \(\mathrm{CaSO_{4} \cdot \tfrac{1}{2}H_{2}O}\), that is one water molecule for every two units of calcium sulphate, also written \(\mathrm{(CaSO_{4})_{2} \cdot H_{2}O}\). It is made by heating gypsum, \(\mathrm{CaSO_{4} \cdot 2H_{2}O}\), to about 373 K.
As printed, the keyed option combines the doubled sulphate with the half molecule of water, but it is the only option describing the hemihydrate. The options with '2HO' are not valid formulas, and the one with a third of a water molecule is not plaster of Paris.
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