The standard solution of FeSCN2+ (prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 w/1.00 mL of 0.0020 M KSCN) has an absorbance of 0.520. If a trial's absorbance is measured to be 0.275 and its initial concentration of SCN– was 0.00060 M, the equilibrium concentration of SCN– will be????

Question

The standard solution of FeSCN2+  (prepared by combining 9.00 mL of 0.200 M Fe(NO3)3  w/1.00 mL of 0.0020 M KSCN) has an absorbance of 0.520. If a trial's absorbance is measured to be 0.275 and its initial concentration of SCN–  was 0.00060 M, the equilibrium concentration of SCN–  will be????

i think i have to use mv=mv, but i don't know how to relate absorbance on this problem.

DrBob222 · Answer

The Beer-Lambert Law.
A = abc.
Use A = abc for the standard to calculate the constant, a (the absorptivity). b = cell length (but you don't need to use that IF both standard and unknown were measured using the same cell. Then use A= abc on the unknown to calculate FeSCN^+2 in the final solution. Knowing how much that is will let you determine the amount of KSCN remaining unreacted in the final solution. Notice that you obtain c (the concn of the standard by mols/L = M. Note also that the standard solution is prepared and you must calcualte molarity; i.e., it isn't 0.200 M. Post your work if you get stuck.