What is the effect of addition of oxalic acid and `HgCl_2` to the following equilibrium
`F_(aq)^(3+)+ SCN_(aq)^- leftrightarrow [Fe(SCN)]_(aq)^(2+)`
Yellow Colourless Deepred

Explanation

Oxalic acid reacts with `Fe^(3+)` ions to form the stable complex ion `[Fe(C_2O_4)_3]^(3-)`.
The formation of stable complex decreases the concentration of free `Fe^(3+)` ions . Hence backward reaction favours and decreases the intensity of red colour.
`HgCl_2` also decreases the intensity of red colour because `Hg^(2+)` reacts with `SCN^(-)` ions to form stable complex in `[Hg(SCN)_4]^(2-)`.