IIT JEE Advanced 2009 Paper 1

The compound X is H2S:

Na2S

(X)

+2H+ → H2S+2Na+

Thus, the compound Y is FeCl3.
Compound Y on reaction with potassium hexacyanoferrate(II) forms intense blue precipitate which dissolves on addition of reagent
4FeCl3+3K4[Fe(II)(CN)6] →

Fe4[Fe(CN)6]3

Intense‌blue‌ppt.

+12KCl
Compound Y on reaction with potassium hexacyanoferrate (III) forms brown colouration due to
FeCl3+K3[Fe(III)(CN)6] →

Fe[Fe(CN)6]

Brown‌colouration−Z

+3KCl