Question

What is the formula of dead burnt plaster?

Answer 1

The formula of any compound is the representation of all the atoms of the element of that molecule in their respective quantities. Plaster is a compound that has the property to settle and become hard, it is calcium sulfate hemihydrate. This when heated becomes dead burnt plaster.

Complete answer:
We have to find the chemical formula of dead burnt plaster. As we know a common setting substance called plaster of Paris is prepared by the heating of gypsum, which is calcium sulfate dihydrate. The reaction for which is:
$CaS{{O}_{4}}\cdot 2{{H}_{2}}O\xrightarrow{373\,K}CaS{{O}_{4}}.{}^{1}/{}_{2}{{H}_{2}}O+\frac{1}{2}{{H}_{2}}O$
The compound formed is plaster of Paris that is calcium sulfate hemihydrate, $CaS{{O}_{4}}.{}^{1}/{}_{2}{{H}_{2}}O$. The one and a half water molecule denotes the water of crystallization that is present in the crystal lattice of the compound. When this plaster of Paris, which is calcium sulfate hemihydrate is heated at a high temperature above 393 K, then the calcium sulfate hemihydrate loses water of crystallization to form an anhydrous salt of calcium sulfate. This anhydrous salt is called dead burnt plaster. The reaction for which is:
$CaS{{O}_{4}}\cdot {}^{1}/{}_{2}{{H}_{2}}O\xrightarrow{473\,K}CaS{{O}_{4}}+{}^{1}/{}_{2}{{H}_{2}}O$
Hence, dead burnt plaster, which is anhydrous calcium sulfate, has the formula $CaS{{O}_{4}}$.

Note:
Plaster of Paris has remarkable property to settle with water due to which it is used as a decorative material. It is used in settling of plasters for the fractured bones, and for making statues. Dead burnt plaster is a more hard and rigid form as it does not have any water of crystallization.