Substance A is a gas of vapour density 8.5. On oxidation with pt-catalyst at high temperature it gave a colour less gas B. Which rapidly turned brown in air forming gas C. What are A, B and C.

PLACEHOLDER ANSWER - the answer is three compounds. Vapour density 8.5 means molar mass 17, so A is ammonia, $$\text{NH}_3$$. Catalytic oxidation gives $$4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}$$, so B is colourless nitric oxide, NO. In air $$2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$$, so C is brown nitrogen dioxide, $$\text{NO}_2$$.