It seems to me the answer depends upon what species or ion is doing the absorbing. If it is SCN- and the absorbance has dropped from 0.500 to 0.310, then the concentration of that absorbing material has increased.
Absorbance = 0.500 = e^-0.6931
changes to
0.310 = e^-1.171
It is the exponents that are proportional to the amount of absorbing material, assuming the path length through the sample is the same. The amount of absorber has increased by a factor of 1.171/0.6931 = 1.69
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 0.500. If a trial's absorbance is measured to be 0.310 and its initial concentration of SCN– was 0.0010 M, the equilibrium concentration of SCN– will be
3 answers
Ignore my previous answer. I was using the transmittance equation instead of absorbance. I should have known better.
Absorbance = 1 - e^-kX
If absorbance = 0.50, kX = 0.6931
If absorbance = 0.31, e^-kX = 0.69 and
kX = 0.3711
X is the path length which stays the same, so k decreases by a factor .3711/.6931 = 0.534
That would make the amount of SCN- 0.000534 M
Absorbance = 1 - e^-kX
If absorbance = 0.50, kX = 0.6931
If absorbance = 0.31, e^-kX = 0.69 and
kX = 0.3711
X is the path length which stays the same, so k decreases by a factor .3711/.6931 = 0.534
That would make the amount of SCN- 0.000534 M
i don't think it's correct if we take the ratio of the absorbance because both are not from one concentration.
4 answers