If 8 cm3 of 9.9 mol m-3 Fe(NO3)3 are mixed with 10 cm3 of 1.0 mol m-3 KSCN solution in a 20 cm3 volumetric flask and water is added to bring the total volume up to 20 cm3, what is the concentration of the Fe3+ ions (in mol m-3)?

Answer 1

You know Fe^3+ forms the FeSCN^2+ complex. What are the units for cc x m^-3? millimols/m^3?
8 x 9.9 = 79.2
10 x 1 = 10
.........Fe^3+ + SCN^- ==> FeSCN^2+
I.......79.2.....10.........0
C......-10......-10........10
E.......69.2......0........10

Keq for this reaction is relatively large; we assume all of the "free" Fe^3+ is due to that which has not reacted. Thus the concn Fe^3+ is
69.2 mmols x (8 cc/20 cc) = ? mol/m^3