Question

Which of the following is the correct formula of Gypsum?
A. $CuS{O_4}.2{H_2}O$
B. $CaS{O_4}.\dfrac{1}{2}{H_2}O$
C. $CuS{O_4}.5{H_2}O$
D. $CaS{O_4}.2{H_2}O$

Answer 1

Hint: Gypsum is a compound of calcium. It is used as a fertilizer and one of the most common minerals in a sedimentary environment.

Complete step by step solution:
Gypsum is a soft sulphate mineral, which consists of dihydrate calcium sulphate. Its chemical formula is $CaS{O_4}.2{H_2}O$. It is widely used as a fertilizer. It is found in layered sedimentary deposits. It is very similar to anhydride ($CaS{O_4}$). Anhydride, upon hydration gives back gypsum. It is also the most common sulphate mineral.
The chemical reaction involving preparation of gypsum is:
$CaC{O_3}_{(s)} + {H_2}S{O_4}_{(aq)} \to CaS{O_4}_{(s)} + C{O_{2(g)}} + {H_2}{O_{(l)}}$
$Ca{(OH)_{2(aq)}} + {H_2}S{O_4}_{(aq)} \to CaS{O_4}_{(s)} + 2{H_2}{O_{(l)}}$
$CaS{O_4}_{(s)} + 2{H_2}{O_{(l)}} \to CaS{O_4}.2{H_2}O$
Gypsum can be prepared by the action of dilute sulphuric acid on calcium carbonate or on calcium hydroxide. It is a white crystalline solid, sparingly soluble in water.
Gypsum, on heating at different temperatures, gives different products.
On heating at 393K, it gives $CaS{O_4}.\dfrac{1}{2}{H_2}O$, which is Plaster of Paris.
$CaS{O_4}.2{H_2}O \to CaS{O_4}.\dfrac{1}{2}{H_2}O$
On heating at 437K and above, it gives anhydrous calcium sulphate or anhydride.
$CaS{O_4}.2{H_2}O \to CaS{O_4}$

Hence, the correct answer is (D) $CaS{O_4}.2{H_2}O$.

Additional information:
Some of the uses of gypsum are:
It is used in the manufacturing of cement.
It is used as a drying agent.
It is also used to prepare ammonia, bleaching powder and calcium nitrate.

Note: A student can most probably confuse between Plaster of Paris ($CaS{O_4}.\dfrac{1}{2}{H_2}O$) and Gypsum ($CaS{O_4}.2{H_2}O$). Both differ in the water of hydration. Make sure to remember Plaster of Paris has less water molecules of hydration.