If a 0.10 M solution of a colored substance has a maximum absorbance at 500 nm and an absorbance of 0.26 M at this wavelength, what will be the measured absorbance of a 0.20 M solution at 500 nm?

Question

aaronq · Answer

absorbance is proportional to conc.

use the Beer-Lambert law:  Absorbance = concentration*path length*$\epsilon$

anonymous · Answer

i never learned the beer lambert law. so how would i plug it in? is c 0.20 M? and l= 500 nm? and then what is that e symbol? 
sorry i'm so confused!

aaronq · Answer

$\epsilon$ is the molar absorptivity coefficient  which is dependent on wavelength, since both measurements are taken at 500 nm, you can ignore it. Same with path length, (the cuvette used was probably the same one).

just use a ratio:    $\dfrac{A_1}{C_1}=\dfrac{A_2}{C_2}$

anonymous · Answer

okay, so i kind of get it. in this problem, A1= 0.26 M, C=0.10 M, C2=0.20 M, and I am trying to find A2. is that correct? and the reason why we are ignoring the cuvette, is because it's just the same (500 nm)? 
sorry again, i just want to make sure i get it >_<

aaronq · Answer

yeah, you're finding A2.

The reason were ignoring $\epsilon$ is because both measurements were taken at the same wavelength (500 nm), the cuvette determines the path length which would be the same. We ignore constants when comparing them.

anonymous · Answer

oh okay. THANK YOU SO MUCH!

aaronq · Answer

no problem !