Question

What is the chemical formula for plaster of Paris? How is it prepared? State the common and the chemical names of the compound formed when plaster of Paris is mixed with water.

Answer 1

The chemical formula for plaster of paris is calcium sulphate hemihydrate $({\mathrm{CaSO}}_4.\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{H}}_2\mathrm{O})$.
Plaster of Paris is prepared by heating of gypsum at 373 K.

$2{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\stackrel{373\mathrm{K}}{\to }\underset{\mathrm{Plaster}\mathrm{of}\mathrm{paris}}{2{\mathrm{CaSO}}_4.\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{H}}_2\mathrm{O}}+3{\mathrm{H}}_2\mathrm{O}$

When we mix plaster of Paris with the appropriate amount of water, it will give gypsum chemically known as calcium sulphate dihydrate.

$2{\mathrm{CaSO}}_4.\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{H}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}\to \underset{\mathrm{Gypsum}}{2{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}}$