Question

Answer the following:
(a) Why is 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 CaSO₄?
(b) Why is sodium hydrogencarbonate an essential ingredient in antacids?
(c) When electricity is passed through an aqueous solution of sodium chloride, three products are obtained. Why is the process called chlor-alkali?

Answer 1

(a) Calcium sulphate hemihydrate or plaster of paris is formed when gypsum is heated at a temperature of 373K.
${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\stackrel{∆}{\to }{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$
Plaster of paris is written in this form to show that two molecules of calcium sulphate combine with one molecule of water. Thus, each calcium sulphate molecule gets only half the share of the water molecule and hence, hemihydrates.

(b) The excess of acid in the stomach causes acidity. As sodium bicarbonate is alkaline in nature, it neutralizes the excess acid and relieves discomfort. Hence, it is an essential ingredient in antacids.
${\mathrm{NaHCO}}_3+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$

(c) When electricity is passed through an aqueous solution of sodium chloride it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas.
$2\mathrm{NaCl}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{NaOH}\left(\mathrm{aq}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

This process is called the chlor-alkali process because of the products formed chlor- for chlorine and alkali for sodium hydroxide.