[azurken]

A post-lab homework problem I'm stuck on.

FeSCN²⁺ becomes visible when the SCN^- concentration reaches appx. 10^-6
What is the concentration of Ag+ in solution at the endpoint, given that Ksp of AgSCN is around 10^-6?

Calculate the percent error this will introduce into your calculation of the value of the Ksp of silver acetate in mixture 1. To do this calculation, you will need to find the actual concentration of Ag+ in mixture 1, which equals the molarity you obtained from your titration plus the molarity of Ag+ remaining at the endpoint, corrected for dilution.

My data:

I have no idea what that last part meant. Which I don't understand.

here is my data for mixture 1

[Ag+] = 0.098 M at eqbm
[C2H3O2-] = 0.037 M at eqbm
Ksp = 3.6E-3

Original/initial soln was:
13.2 mL of 0.200M AgNO3 + 7.0 mL of 0.200 M sodium acetate
[Ag+] = 0.131 M
[acetate] = 0.070 M

10.0 ml Of this soln was titrated with 4.19 mL of 0.1025 KSCN-
used 2

Ag reacts with SCN

 used was 20 drops of 2M HNO3 and 15 drops of Fe(III) (the indicator)

Anybody have a clue what its really asking?
I don't understand what the "actual" concentration is. How is that different from the eqbm molarity? I'm guess the indicator or those drops account as something for the end point?

[azurken]

Okay, I can't find the edit button, but this question is essentially saying that even at the end point of the titration, there is some Ag+ left (even though theoretically it should have all been titrated by the SCN-). This is the part where I don't understand on how do I calculate this number?

[Borek]

Okay, I can't find the edit button, but this question is essentially saying that even at the end point of the titration, there is some Ag+ left (even though theoretically it should have all been titrated by the SCN-).

End point is when you added stoichiometric amount of SCN^-. Technically it means you have just a saturated solution in contact with the solid. Because of the way solubility works, you can't "titrate all" - some Ag⁺ will be always present in the solution.

This is the part where I don't understand on how do I calculate this number?

Think about solubility product.