Help with equilibrium concentrations?

Question

we mixed 5mL of .002 M Fe(Cl3)3 and 5mL of .002 M KSCN.

absorbance=0.628

Fe3+(aq) + 2SCN- <~> Fe(SCN)2

1. calculate initial concentration of fe3+ and SCN- ions in the solution.

2. Calculate the Keq of Fe(SCN) in the sol'n.

Answer 1

1. Since you mixed equal volumes, you cut the initial concentration of each reactant in half..  0.001M FeCl3 and 0.001M KSCN

2.  A = abc, where A = absorbance, a = molar absorptivity, b = path length, and c = concentration.

In order to use the absorbance to get the concentration we need to know the molar absorptivity and the path length (which is usually 1 cm).

Then you can determine the concentration of Fe(SCN)2 and Keq.

Keq = [Fe(SCN)2] / [Fe3+] [SCN-]^2