Plaster of Paris should be stored in a moisture-proof container. Explain why?

Which of the following is the correct formula of plaster of Paris?
${\mathrm{CaSO}}_4.{\mathrm{H}}_2\mathrm{O}$  or  $2\left({\mathrm{CaSO}}_4\right).{\mathrm{H}}_2\mathrm{O}$

Which of the following properties are shown by dilute $\mathrm{HCl}$?

(i) It turns blue litmus red. 
(ii) It turns red litmus blue.
(iii) It reacts with zinc and a gas is evolved. 
(iv) It reacts with solid sodium carbonate to give brisk effervescence.