# Homework Help: Spectrophotometry problem

1. Sep 4, 2007

### Benzoate

1. The problem statement, all variables and given/known data

If a 0.390 mM solution of MnO41- has an absorbance of 0.585 at 525 nm in a 1.000 cm cell. What is the concentration of a MnO41- solution that has absorbance of 0.330 in the same cell at that wavelength?

2. Relevant equations

3. The attempt at a solution

c(1) =concentration =.390 *10^-9 M
wavelength = 525*10^-9 m
pathlength = 1*10^-2 m
A(1)= absorbance = .585
A(2) = .330

I think the wavelength and pathlength are irrelevant to finding the concentration of (MnO4)1-

A(1)/A(2)= c(1)/c(2) => c(2)=c(1)*A(2)/A(1)

C(2)= (.330) *(.390*10^-9)/(.585) = 2.20*10^-9

Last edited: Sep 4, 2007
2. Sep 4, 2007

### symbolipoint

You have it; at least symbolically. Your constants substituted look good also.
...
Now, how do you know that the concentration and absorbance are in a linear relationship (they probably are.)?

3. Sep 5, 2007

### chemisttree

It looks good except that a millimolar (mM) is 10^-3 moles/liter not 10^-9 moles/liter.