A white salt X: (i) warmed with copper turnings and concentrated $H X_{2} SO X_{4}$ gives a brown gas and a blue solution; (ii) warmed with aqueous NaOH gives a gas turning moist red litmus blue; (iii) heated alone carefully to about 250 $^{\circ}$ C leaves no residue and gives a colourless gas that rekindles a glowing splinter. Identify X and the oxidation state of nitrogen in the gas of (iii).

Answer: A

Test (i) shows nitrate, test (ii) ammonium, so X is

NH X_{4} NO X_{3}

. Careful heating gives

NH X_{4} NO X_{3} N X_{2} O + 2 H X_{2} O

and

N X_{2} O

supports combustion. In

N X_{2} O

the oxidation state of N is +1 (

NH X_{4} NO X_{2}

would give

N X_{2}

, which does not rekindle a splinter).