On adding two or three drops of `FeCl_(3)` into the excess of solution (A) a violet coloured compound (D) is formed. This colour disapperars quickly.

Answer

The reactions indicate that the compound (A) is sodium thiosulphate. It is formed in step (i) by passing gas (B) which is either `I_(2)`.
`S_(2)O_(3)^(2-)+Fe^(3+) underset((D)" violet")(rarr) [Fe^(3+)(S_(2)O_(3))_(2)]^(-)`
`[Fe(S_(2)O_(3))_(2)]^(-)+Fe^(3+) rarr 2 Fe^(2+)+S_(4)O_(6)^(2-)`