Question

What is the chemical formula of Gypsum?
(A) CaSO$_4$.5H$_2$O
(B) CaSO$_4$.7H$_2$O
(C) CaSO$_4$.2H$_2$O
(D) CaSO$_4$.9H$_2$O

Answer 1

Hint: Gypsum is a type of mineral composed of sulphur, oxygen, calcium, and the water molecules. It is named as the calcium sulphate dihydrate. With the help of chemical reaction, we can determine the exact chemical formula of gypsum.

Complete step by step answer:

• First, Gypsum is mainly found from lake and sea water in the form of sedimentary rocks. It is a soft crystalline mineral.
• It is formed by the evaporation process.
• We know that the sea water, and lake has the content of calcium, sulphur, oxygen, and many more elements, but when the evaporation occurs sulphur bonds with the oxygen leading to the formation of a sulphate SO$_4$ $^{2-}$.
• Then, further the reaction proceeds, and the sulphate bonds with the calcium, and the water molecules.
• The chemical reaction is Ca $^{2+}$ + (SO$_4$)$^{2-}$ + 2H$_2$O $\rightleftharpoons$ CaSO$_4$.2H$_2$O
  Now, this reaction leads to the formation of a gypsum.
  From the chemical reaction we can say that there are 2 molecules of water.
  Thus, the chemical formula of gypsum is CaSO$_4$.2H$_2$O. The correct option is C.

Additional information:
Gypsum is used as a fertilizer, or mainly in the plaster, chalk, and drywall.

Note: Carefully write down the chemical equation of formation of gypsum and balance it. Then we can identify the presence of water molecules present in it.