Question

Give the chemical formula of calcium sulphate hemihydrate.
A. ${\text{CaS}}{{\text{O}}_4}.2{{\text{H}}_2}{\text{O}}$
B. ${\text{CaS}}{{\text{O}}_4}.\dfrac{1}{2}{{\text{H}}_2}{\text{O}}$
C. ${\text{CaS}}{{\text{O}}_4}.10{{\text{H}}_2}{\text{O}}$
D. ${\text{CaS}}{{\text{O}}_4}.{{\text{H}}_2}{\text{O}}$

Answer 1

Hemihydrate means the molecules with half mole of water in it. Means in one mole of molecules only half a mole of water that is ${{\text{H}}_{\text{2}}}{\text{O}}$ is present. Use this concept to find the answer.

Complete step by step answer:
Let’s start discussing hemihydrate to get closer to the answer. Hemihydrate means the molecules with half mole of water in it. Means in one mole of molecules only half a mole of water that is ${{\text{H}}_{\text{2}}}{\text{O}}$ is present.
Coming to the question we are provided with the molecule calcium sulphate hemihydrate. The chemical formula of calcium sulphate is ${\text{CaS}}{{\text{O}}_{\text{4}}}$ and by the definition and understanding of hemihydrate, it means $\dfrac{1}{2}{{\text{H}}_2}{\text{O}}$, hence the molecular formula of calcium sulphate hemihydrate is${\text{CaS}}{{\text{O}}_4}.\dfrac{1}{2}{{\text{H}}_2}{\text{O}}$.
So, the answer of this question is option B. ${\text{CaS}}{{\text{O}}_4}.\dfrac{1}{2}{{\text{H}}_2}{\text{O}}$.

$\therefore$ Option B is correct.

Note:
We also remember that Calcium sulphate hemihydrate, popularly known as plaster of Paris is one of the most used materials by the constructors and architects. Plaster of Paris is being used as a building material from last $5000$ years and earlier was used by Egyptians to decorate the tombs. It forms a paste when mixed with water and when it dries, it gives a solid texture and hardens and hence is used as a building material.