Question

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.

Answer 1

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

Answer 2

Describe how you could separate the components of a mixture of PbCl2 and NaSO4 and recover the two substances.