Green crystals Z lose water on gentle heating to give a white solid, which on strong heating leaves a red-brown residue and evolves a gas mixture. The gases are passed through acidified $BaCl X_{2}$ solution. Atomic masses: Ba 137, S 32, O 16. The mass of white precipitate formed per mole of Z used is

Answer: D

Z is

FeSO X_{4} \cdot 7 H X_{2} O

. Strong heating:

2 FeSO X_{4} Fe X_{2} O X_{3} + SO X_{2} + SO X_{3}

. Only

SO X_{3}

(as

H X_{2} SO X_{4}

) precipitates

BaSO X_{4}

in acid, because

BaSO X_{3}

dissolves. 1 mol FeSO4 gives 0.5 mol

SO X_{3}

, so 0.5 x 233 = 116.5 g of

BaSO X_{4}

(M = 137 + 32 + 64 = 233).