What is the chemical formula of plaster of paris ?
CaSO4. 1/2 H2O.
Calcium sulphate with half a molecule of water per molecule of the salt (hemi-hydrate) is called plaster of paris (plaster of paris).
It is prepared by heating gypsum (CaSO4.2H2O) at 120°C in rotary kilns, where it gets partially dehydrated