A colourless sodium salt X, when warmed with concentrated H2SO4, gives violet vapours and also a gas that blackens lead acetate paper. Its aqueous solution gives a yellow precipitate with AgNO3 that is insoluble in concentrated ammonia. Identify X and the moles of the violet substance formed per mole of H2SO4 reduced to the gas that blackens lead acetate paper.

JEE Advanced levelSingle correct optionDifficulty 3 of 3qa142

A colourless sodium salt X, when warmed with concentrated $H X_{2} SO X_{4}$ , gives violet vapours and also a gas that blackens lead acetate paper. Its aqueous solution gives a yellow precipitate with $AgNO X_{3}$ that is insoluble in concentrated ammonia. Identify X and the moles of the violet substance formed per mole of $H X_{2} SO X_{4}$ reduced to the gas that blackens lead acetate paper.

A yellow

AgI

precipitate insoluble in conc. ammonia and violet vapour identify iodide. The gas is

H X_{2} S

8 HI + H X_{2} SO X_{4} 4 I X_{2} + H X_{2} S + 4 H X_{2} O

. Sulphur gains 8 electrons (+6 to -2) and 8 iodide ions lose one each, so 4 mol

I X_{2}

form per mole of acid reduced.