1. (i) The element nitrogen can be obtained in the pure state by removing carbon dioxide and oxygen from air.   2. (ii) Ammonium salts will, on heating, decompose to give ammonia.   3. (iii) Lead chloride can be prepared by adding dilute hydrochloric acid to lead sulphate solution.   4. (iv) 1 gram of any gas occupies 22.4 litres at S.T.P.