Question

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

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 (A) \[CaS{O_4}.2{H_2}O\].

Additional information:
Some of the uses of gypsum are:
1. It is used in the manufacturing of cement.
2. It is used as a drying agent.
3. 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.