What's the total volume? That's 0.50 + 2.50 + 2.00 mL = 5.00 mL.
How many mols FeSCN^2+ do you have? That's mols = M x L = 1.25E-4 x 0.0005 = ? (You're assuming here that the formation constant for FeSCN^2+ is so large that the reaction goes so far to the right that there is no Fe(NO3)3 left and all has been converted to the complex ion). Of course the Fe(NO3)3 is the limiting reagent.
Finally, M = mols/L. You have total mols and total L. Calculate M of the complex ion.
So there's this prelab assignment we're supposed to complete where we would fill out this table with [FeSCN^2+] in each of the test tubes. I'm not completely sure on how to do this?
For example, on the first tube calculate the molarity of [FeSCN^2+] with the data given:
-Volume of 1.25x10^-4M Fe(NO3)3 mL: 0.50
-Volume of 1.0M KSCN mL: 2.50
-Volume of 0.10M HNO3 mL: 2.00
-Calculated [FeSCN^2+] M: ???
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