An ore X, heated in excess of air, gives a colourless pungent gas Y that turns acidified $K X_{2} Cr X_{2} O X_{7}$ paper green, and a solid Z. Z is reduced by carbon only near 1673 K, the metal being obtained as vapour that is condensed. The identity of X and the number of moles of Y per mole of X are

Answer: C

The gas turning dichromate green is

SO X_{2}

, so X is a sulphide. A metal that is a vapour at 1673 K obtained from its oxide by carbon is zinc, so X is

ZnS

with

2 ZnS + 3 O X_{2} 2 ZnO + 2 SO X_{2}

: 1 mol

SO X_{2}

per mol

ZnS

ZnCO X_{3}

would give

CO X_{2}

and

FeS X_{2}

gives iron, which is not a vapour.