Barium sulphate obtained from a sulphate is heated strongly with carbon: $BaSO X_{4} + 4 C BaS + 4 CO$ . The product is dissolved in dilute HCl and the gas evolved is tested with filter paper moistened with lead acetate. The number of electrons gained by sulphur per formula unit of $BaSO X_{4}$ , and the colour of the lead compound formed on the paper, are respectively

Answer: A

Sulphur changes from

+ 6

BaSO X_{4}

- 2

BaS

, a gain of 8 electrons (carbon goes from 0 to

+ 2

, 4 atoms giving 8 electrons).

BaS + 2 HCl BaCl X_{2} + H X_{2} S

and

H X_{2} S

gives black

PbS

on lead acetate paper. The answer is 8 and black.