[Mace1988]

We had an experiment in my chem 121 class where we used a spectroscope with different solutions to determine the absorbance. We found max absorbance for certain solutions, and are supposed to make a graph of absorbance vs. wavelength in excel. The second part is to find the concentration, molar absorptivity, and the concentration of an unknown solution we measured. I can't figure out how to find the concentration or molar absorptivity. I know the A=ebc formula but I need either concentration or molar absorptivity to find the other.

[fledarmus]

Could you be more specific about the "different solutions" that you used to determine the absorbance? Were those different concentrations of the same compound, or were they all different compounds at some unknown concentration?

[Mace1988]

Yes, the three solutions were Ni²⁺, Cu²⁺, Co²⁺. We measured the absorbance at wavelengths from 360-780nm, measuring each 20 nm. I'm responsible for .24 M Ni²⁺, and found its max absorbance to be 400nm. Once that was found we diluted the .24M nickel nitrate solution 4 times by adding 5mL of distilled water into sample, stirring, taking 5mL of that out into a new test tube, and repeating. We got the absorbance of the 4 diluted solutions, and that's as far as I've gotten.

[fledarmus]

.24 M Ni²⁺,

Ah, then you already know the concentration of your standard! Then you did the sample dilutions and you know what volume you diluted them to, so you can calculate the concentration of each of those samples. You measured the absorbance of each concentration, so you can prepare a chart of concentration against absorbance.

[Mace1988]

Oh wow i was really over thinking it. So my concentration would be cut in half for each subsequent dilution?