A white crystalline solid X crackles (decrepitates) on heating, evolving a brown gas and a colourless gas that rekindles a glowing splinter. The residue is yellow even when cold. Heated on charcoal with Na2CO3, the residue gives a soft, malleable grey bead that marks paper and a yellow incrustation. Identify X and the mole ratio of brown gas to colourless gas evolved.

JEE Advanced levelSingle correct optionDifficulty 3 of 3qa125

A white crystalline solid X crackles (decrepitates) on heating, evolving a brown gas and a colourless gas that rekindles a glowing splinter. The residue is yellow even when cold. Heated on charcoal with $Na X_{2} CO X_{3}$ , the residue gives a soft, malleable grey bead that marks paper and a yellow incrustation. Identify X and the mole ratio of brown gas to colourless gas evolved.

The soft, malleable, paper-marking bead with a yellow incrustation is lead, so the residue is

PbO

and X is

Pb (NO X_{3}) X_{2}

(bismuth gives a brittle bead, silver leaves white metal directly).

2 Pb (NO X_{3}) X_{2} 2 PbO + 4 NO X_{2} + O X_{2}

gives

NO X_{2} : O X_{2} = 4 : 1