A student determines K for Fe^3+ + SCN- <-> FeSCN^2+ by spectrophotometry at 447 nm. A standard curve made with [SCN-] in large excess gives epsilon = 4630 M^-1 cm^-1 (1.00 cm path). In the equilibrium trial, the student mixes equal volumes to give initial [Fe^3+] = 2.00e-3 M and initial [SCN-] = 2.00e-3 M (after dilution). The measured equilibrium absorbance is A = 1.85.
What is the equilibrium constant K for Fe^3+ + SCN- <-> FeSCN^2+?
- A
4.0e-4
- B
200
- C
8.0e-7
- Dcheck_circle
1.6e2
Explanation
[FeSCN^2+]_eq = 4.0e-4 M. [Fe^3+]_eq = [SCN-]_eq = 2.00e-3 - 4.0e-4 = 1.6e-3 M each. K = 4.0e-4 / (1.6e-3)^2 = 4.0e-4/2.56e-6 = 156 ~ 1.6e2.