Q2.why is plaster of paris witten as CaSO4.1/2 H 2 O? How is it possible to have half a water molecule attached to CaSO4?

Give any two important uses of this compound.

The chemical formula of plaster of Paris is CaSO4.1/2H2O. This chemical formula basically indicates that two units of calcium sulphate share one molecule of water. The uses of this compound are as follows:

1) It is used by doctors as plaster for supporting fractured bones in the right position.

2) It is also used for making toys, materials for decoration and for making surfaces smooth.

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When hydrated calcium sulphate (CaSO4.2H2O) or gypsum is heated at 373K, it loses its water molecules and forms calcium sulphate hemihydrate (). This hemihydrated form of calcium sulphate is known as Plaster of Paris. It is in the form of a white powder.

When the powder of Plaster of Paris is mixed with water, it becomes hard and solid gypsum. Plaster of Paris is generally used to support fractured bones in their correct positions.

In, only half a water molecule is shown as the water of crystallization because two molecules of CaSO4 share one molecule of water.

Plaster of Paris is used for making toys, materials for decoration, and for making smooth surfaces.

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 calcium sulphate hemihydrate (CaSo4.1/2H2O)  or plaster of paris is formed when gypsum ( CaSo4.2H2O) is heated at a temperature of 373 K.

CaSo4.2H2O ---heat----> CaSo4.1/2H2O + 1 1/2H2O

CaSo4.1/2H2O is written in this form to show that 2 molecules of CaSo4 combine with 1 molecule of water. Thus each calcium sulphate molecule get only half the share of water molecule and hence hemihydrates.

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gypsum (CaSO4.2H2O)  when heated at 373 k it loses 3/4 molecule of the water and decomposes to give plaster of parins (CaSO4.1/2H2O)

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