Question

The dead burnt plaster is:
(A) CaSO$_4$.2H$_2$O
(B) CaSO$_4$.$\dfrac{1}{2}$H$_2$O
(C) CaSO$_4$
(D) 2CaSO$_4$.H$_2$O

Answer 1

Hint: We have to identify the chemical formula of dead burnt plaster. From the name itself we can say that it is relatable to the plaster of Paris. Consider the reaction of heating the plaster of Paris.

Complete step by step answer:
> Now, we will know about the dead burnt plaster. As mentioned it is relatable to the plaster of Paris.
So, let us first know the formation of plaster of Paris. It is formed by the heating of gypsum.
> The chemical reaction can be written as:
CaSO$_4$.2H$_2$O + heat $\rightarrow$ CaSO$_4$.$\dfrac{1}{2}$H$_2$O+1.5H$_2$O
Here, the product obtained is plaster of Paris attained by heating at the 423 K.
As we know, the plaster of Paris is also known as calcium sulphate hemihydrate.
> Now, if we heat the plaster of Paris, the chemical reaction:
CaSO$_4$.$\dfrac{1}{2}$H$_2$O $\xrightarrow{200^\circ C}$ CaSO$_4$ $\xrightarrow{1100^\circ C}$ CaO+SO$_3$
Here, in this reaction we can see that plaster of Paris is being heated, and the product attained is anhydrous calcium sulphate.
> We can see the second step too in which there is formation of calcium oxide.
So, by seeing the reaction, we got to know that in the intermediate product, i.e. anhydrous calcium sulphate, there is loss of water molecules.
Thus, we can say that anhydrous calcium sulphate i.e. CaSO$_4$ is the dead burnt plaster.
Hence, the correct option is (C).

Note: Don’t get confused about why we considered anhydrous calcium sulphate as the dead burnt plaster. As mentioned it has absorbed the water molecules, and it becomes anhydrous molecule. So it is considered to be the dead burnt plaster.