how is Fe{3+} affected when you ADD HPO4{2+}?

Question

Solution: Fe{3+}(aq) + SCN{-}(aq) —> Fe(SCN){2+}(aq)

I know that the color gets clear when you add HPO4{2+}, but I don’t know why.

Answer 1

Lauren's answer is essentially correct, but I will add additional information to hopefully give you a cogent explanation.

Fe³⁺(aq) + SCN^-(aq) <--> Fe(SCN)²⁺(aq)  ... reaction 1

2Fe³⁺(aq) + 3HPO₄^-(aq) <--> Fe₂(HPO₄)₃(s)  ... reaction 2

Note that in reaction 2, there is the formation of a precipitate, which essentially serves to remove Fe from solution.  According to Le Chatelier's Principle, this will force reaction 1 to the left, since reactant (Fe³⁺) is being removed.  This shift in equilibrium is reflected by the color change from brownish-red (the color of Fe(SCN)²⁺ to clear.

I hope this explains why you see the color change.