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 concentration of FeSCN^2+?
- A
2.3e-7 M
- B
1.0e-3 M
- C
1.85e-3 M
- Dcheck_circle
4.0e-4 M
Explanation
[FeSCN^2+] = A / (epsilon * b) = 1.85 / (4630 * 1.00) = 4.0e-4 M.