[stewstew]

In the Volhard titration used in this experiment, a trace of Fe3+ is added to the excess Ag+ prior to titration with KNCS. Once all of the excess Ag+ has reacted, the red complex of Fe(NCS)2+ forms. This red complex is visible when the [NCS-] is about 2x10^-4M. At this point > 99% (but not all) of the Ag+ has precipitated. Calculate the [Ag+] when the Fe(NCS)2+ forms.

AgCl(s) -> Ag+ + Cl-          Ksp = 1 x 10^-10
AgNCS(s) -> Ag+ + NCS-    Ksp = 1 x 10^-12
Fe3+ + NCS- -> FeNCS2+   Ksp = 1 x 10^+3

I'm not sure where to begin on this question.  Please help, thanks!

[AWK]

According to forum rules you have to show you attempt

[stewstew]

Oh, sorry!

My first instinct, was to use an ICE chart with the 2nd equation as a guide.
The initial for the NCS- was 2x10-4M with a change of +x
Then I was planned on finding x, using the Ksp value, then substituting in for Ag, but that doesn't work; which, lead me to believe my ICE chart was a failure.

[Borek]

Hmmm, precipitation titration...

You know concentration of SCN^- as it was given to you, why don't you just plug it into AgSCN K_sp?

[stewstew]

Wow! I definitely over complicated things.  Thanks!! The site was really good, too.