A standard FeSCN 2+ solution (0.030 M) has an absorbance = 0.860. What is the co

Question

A standard FeSCN2+ solution (0.030 M) has an absorbance = 0.860. What is the concentration of an unkown FeSCN2+ solution that has an absorbance = 0.596.

Consider the following equilibrium:
Fe3+(aq) + SCN-(aq) FeSCN2+(aq)

Initial concentrations: [Fe3+] = 0.590; [SCN-] = 1.239; [FeSCN2+] = 0

The equilibrium concentration of [FeSCN2+]eq = 0.454 M.

What is the equilibrium concentration of Fe3+? Enter your answer as a decimal number. [Fe3+]eq

Consider the following equilibrium:
Fe3+(aq) + SCN-(aq) FeSCN2+(aq)

Initial concentrations: [Fe3+] = 0.590; [SCN-] = 1.239; [FeSCN2+] = 0

The equilibrium concentration of [FeSCN2+]eq = 0.454 M.

What is the numerical value of KC for this equilibrium? Enter your answer as a decimal number. KC =

Explanation / Answer

(1) Beer's law: A = ecl

where A is absorbance, e is molar absorptivity, c is concentration and l is path length

Since e and l are constant => A is proportional to c

A1 = 0.860, A2 = 0.596

c1 = 0.030 M, c2 = ?

c1/A1 = c2/A2

0.030/0.860 = c2/0.596

Unknown concentration c2 = 0.0208 M = 0.021 M


(2)...Fe3+...+...SCN-...<=>...FeSCN2+

I.....0.590.......1.239................0

C.......-a...........-a.................+a

E...0.590-a....1.239-a................a


At equilibrium:

a = [FeSCN2+] = 0.454 M

[Fe3+] = 0.590 - a = 0.136 M


(3) [SCN-] = 1.239 - a = 0.785

Kc = [FeSCN2+]/[Fe3+][SCN-]

= 0.454/(0.136 x 0.785)

= 4.25

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