Why is POP written as CaSO4.1/2H2O? How is it possible to have half a water molecule attached to CaSO4?
Half a molecule of water means that two units of a chemical substance share one molecule of water. In plaster of Paris, two units of calcium sulphate share one molecule of water, making it 1/2 molecule of water per CaSO4 unit. This is why it is known as calcium sulphate hemihydrate and the chemical formula is CaSO4.1/2H2O.