Identify the correct statement.

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(A) Gypsum is $CaSO_4 \cdot 2H_2O$ and Plaster of Paris is $CaSO_4 \cdot \frac{1}{2}H_2O$. When Plaster of Paris is mixed with water,it rehydrates to

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Gypsum contains a lower percentage of calcium than Plaster of Paris.

Vedclass · Answer

(A) Gypsum is $CaSO_4 \cdot 2H_2O$ and Plaster of Paris is $CaSO_4 \cdot \frac{1}{2}H_2O$. When Plaster of Paris is mixed with water,it rehydrates to form gypsum: $CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$. When gypsum is heated at $373 \ K$,it loses water to form Plaster of Paris: $CaSO_4 \cdot 2H_2O \xrightarrow{373 \ K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$. Comparing the molar masses,the percentage of $Ca$ in gypsum is lower than in Plaster of Paris because of the higher water content in gypsum.

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