Answer the following:
$(i)$ Why is Plaster of Paris written as $CaSO_{4} \cdot 1/2 H_{2}O$? How is it possible to have half a water molecule attached to $CaSO_{4}$?
$(ii)$ Why is sodium hydrogen carbonate an essential ingredient in antacids?
$(iii)$ When electricity is passed through an aqueous solution of sodium chloride,three products are obtained. Why is the process called chlor-alkali?

(N/A) $(i)$ Plaster of Paris is written as $CaSO_{4} \cdot 1/2 H_{2}O$ because two formula units of $CaSO_{4}$ share one molecule of water of crystallization. This means that one water molecule is associated with two $CaSO_{4}$ units,effectively resulting in $1/2 H_{2}O$ per unit.
$(ii)$ Sodium hydrogen carbonate $(NaHCO_{3})$ is a mild,non-corrosive basic salt. It acts as an antacid by neutralizing the excess hydrochloric acid $(HCl)$ produced in the stomach,thereby providing relief from acidity.
$(iii)$ The electrolysis of an aqueous solution of sodium chloride (brine) produces chlorine gas $(Cl_{2})$ at the anode and sodium hydroxide $(NaOH)$,which is an alkali,at the cathode. Due to the formation of these two specific products,the process is termed the chlor-alkali process.