Question

We assumed that all the SCN^- ion was converted to FeSCN^2+ ion in Part 1 because of the great excess (approximately 1000x) of Fe^3+ ion. However, since the equilibrium shown Kf = [FeSCN^2+]/[Fe^3+][SCN^-] takes place, a trace amount of SCN^- ion must also be present.

(a) Use the Kf mean ( 312.56) to calculate the SCN^- ion concentration in solution S3 (8.0e-05M).
(b) Based on your answer in part a, was the assumption made in Part 1 valid? What percentage of SCN^- ion was converted to the FeSCN^2+ ion? Hint: For the assumption to be valid, more than 95% of the SCN^- ion should be converted to FeSCN^2+ ion.

Answer 1

In Part 1, the assumption that all SCN^- ion was converted to FeSCN^2+ ion may not be valid due to the equilibrium constant. To determine the validity of the assumption, we can calculate the SCN^- ion concentration and compare it to the initial concentration.

Step-by-step explanation:

In Part 1, it was assumed that all the SCN^- ion was converted to FeSCN^2+ ion due to the large excess of Fe^3+ ion. However, due to the equilibrium constant Kf = [FeSCN^2+]/[Fe^3+][SCN^-], a trace amount of SCN^- ion must also be present.

(a) To calculate the SCN^- ion concentration in solution S3, we can use the Kf value of 312.56. Since the concentration of FeSCN^2+ is not given, we can assume it is negligible and use the initial concentration of Fe^3+ and SCN^- to calculate the concentration of SCN^- at equilibrium.

(b) Based on the answer in part a, we can determine whether the assumption made in Part 1 was valid. If more than 95% of the SCN^- ion was converted to FeSCN^2+ ion, then the assumption is valid.

Answer 2

The concentration of SCN- in solution S3 can be calculated using the formula [SCN-] = [FeSCN2+]/(Kf × [Fe3+]) with the provided Kf value. The validity of the assumption that almost all SCN- was converted to FeSCN2+ is dependent on the percentage of SCN- that remains unreacted, which is considered acceptable if it is less than 5%.

Step-by-step explanation:

To calculate the SCN- ion concentration in solution S3 given the formation constant (Kf) and the FeSCN2+ concentration, we can set up the equilibrium expression as Kf = [FeSCN2+]/[Fe3+][SCN-]. Rearranging the formula to solve for [SCN-], we get [SCN-] = [FeSCN2+]/(Kf × [Fe3+]). Substituting Kf = 312.56 and [FeSCN2+] = 8.0e-05 M, let's assume [Fe3+] is large and remains practically unchanged due to its great excess; thus, [SCN-] will be approximately the same as the initial FeSCN2+ concentration.

To assess the validity of the assumption that nearly all SCN- was converted to FeSCN2+, we must compare the calculated [SCN-] concentration to the initial [SCN-] concentration. If less than 5% of the SCN- remains unreacted, the assumption is valid. The percentage conversion can be calculated using the initial and equilibrium concentrations of SCN-.