Question

211(a). Why is the formula of plaster of Paris written as ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$? How is it possible to have half a water molecule attached to${\mathrm{CaSO}}_4$?

Answer 1

• The plaster of Paris is calcium sulfate hemihydrate. The formula for Plaster of Paris is ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$.
• Plaster of Paris is prepared from Gypsum.
• Gypsum is calcium sulfate dihydrate $({\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O})$
• Plaster of Paris is prepared by heating Gypsum to a temperature of $100°\mathrm{C}$ in a kiln.
• When gypsum is heated, it loses three fourth of its water of crystallization and forms plaster of Paris.
  ${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{∆}{\to }{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
  
• It is not possible to have half molecule of water.
• The formula ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$ means that two formula units of ${\mathrm{CaSO}}_4$share one molecule of water in such a way that the effective water of crystallization of one particular unit of Calcium sulfate comes to $\frac{1}{2}$ molecule of water.