A gas X from a coal fired power plant turns acidified potassium dichromate paper green and decolourises acidified KMnO4. When X is oxidised and dissolved in rain, the acid formed attacks the marble of a monument. Identify X and the main solid product formed on the marble.

X = NO2; solid product Ca(NO3)2

JEE Advanced levelSingle correct optionDifficulty 3 of 3ev039

A gas X from a coal fired power plant turns acidified potassium dichromate paper green and decolourises acidified $KMnO X_{4}$ . When X is oxidised and dissolved in rain, the acid formed attacks the marble of a monument. Identify X and the main solid product formed on the marble.

Reduction of dichromate (orange to green, Cr(VI) to Cr(III)) and of permanganate shows a reducing gas,

SO X_{2}

(S +4 to +6). It is oxidised to

SO X_{3}

and then

H X_{2} SO X_{4}

in the air and rain.

CaCO X_{3} + H X_{2} SO X_{4} CaSO X_{4} + H X_{2} O + CO X_{2}

gives calcium sulphate.