College Chemistry
posted by Cat .
calculate the initial concentration of SCN ions and Fe3+ ions when a solution was prepared by mixing 3.00mL of 0.00280M KSCN, 6.00 mL of 0.250M Fe(NO3)3 and 16ml of 0.050M HNO3.

You can do this one of several ways. Some are longer than others but all give the same answer. The easy way I think is to just recognize that the initial concn of EACH reagent in the mixture is diluted and solve it that way.
What's the total volume? That's 3.00 + 6.00 mL + 16 mL = 25 mL.
So you diluted KSCN from 3 mL to 25 mL so the new concn is
0.0028M x (3/25) = ?
tough, eh?
Likewise you can calculate the concn of each of the other reagents in the mixture.
If you wish you may use the dilution formula of
c1v1 = c2v2
c = concn
v = volume
0.0028*3 = c2*25
Solve for c2 
Describe how you could separate the components of a mixture of PbCl2 and NaSO4 and recover the two substances.