Question

The difference of water molecules in Gypsum and Plaster of Paris is

• A. 5/2
• B. 2
• C. 1/2
• D. 3/2

Answer 1

The correct option is D

3/2

The explanation for the correct option (D):

• Plaster of Paris is Calcium sulfate hemihydrate. Its chemical formula is (${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$).
• Plaster of Paris is prepared from Gypsum. Gypsum is Calcium sulfate dihydrate (${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$). Therefore, it is Calcium sulfate (${\mathrm{CaSO}}_4$) containing two molecules of water of crystallization.
• Plaster of Paris is produced by heating Gypsum (${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$) to a temperature of 373 K.

$$\begin{gathered} {\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Heat}}{\to }{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Gypsum}\mathrm{Plaster}\mathrm{of}\mathrm{Paris}\mathrm{Water} \end{gathered}$$

• The number of Water molecules in Gypsum is two (2) while the number of Water molecules in Plaster of Paris is half ($\frac{1}{2}$). Thus, the difference in water molecules between Gypsum and Plaster of Paris is:
• $\mathrm{Difference}=2-\frac{1}{2}=\frac{3}{2}$
• Hence, the difference in water molecules between Gypsum and Plaster of Paris is $\frac{3}{2}$.

Final Answer: Hence, option (D) is correct. The difference of water molecules in Gypsum and Plaster of Paris is $\frac{3}{2}$.