Question

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.

Answer 1

(a) (i) $2;1/2$
[1 Mark]
(b) ${\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4.2{\text{H}}_2\text{O}\text{Plaster of Paris}\text{Gypsum}$
[1 Mark]