The chemical formulae of Plaster of Paris and gypsum are _______ and _______ respectively.

Q.

(a) What is plaster of Paris? Write the chemical formula of plaster of Paris.

(b) How is plaster of Paris prepared? Write the chemical equation of the reaction involved.

(c) Explain why plaster of Paris should be stored in a moisture-proof container.

(d) State two important uses of plaster of Paris.

Q.

A lot of dry powder of one of the following chemicals can be released over a fire to extinguish it. This chemical is :

(a) plaster of Paris

(b) baking soda

(c) washing soda

(d) bitumen

Q. Chemically, Plaster of Paris is calcium sulphate hemihydrate, i.e. containing half molecule of water of crystallisation. It is represented by the formula, ${\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O}$. Half molecule of water of crystallisation means that one water molecule is shared by two formula units of $CaS{O}_4$. Hence we also represent its formula as $(CaS{O}_4{)}_2.H_{2}O$. The name, Plaster of Paris, was given to this compound because for the first time, it was made from gypsum which was mainly found in Paris. Gypsum has the chemical formula, $CaS{O}_4.2H_{2}O$. When plaster of Paris is mixed with water it sets into a hard mass.

(a) The number of water molecules present as water of crsytallisation in gypsum and Plaster of Paris respectively, is
(i) $2;1/2$ (ii) $2;5/2$ (iii)$1/2;2$ (iv)$3/2;1/2$

(b) Write the chemical equation for the reaction between plaster of Paris and water.

Q. How many water molecules are present in one formula unit of the following compounds? a) Gypsum b) Plaster of Paris c) Epsom