How can I use Beer's Law to determine the percent of Iron II ion in a sample?

[Lancelot59]

Homework Statement

I'm given the mass of an unknown sample, and the absorbance of said unknown sample. Using that and the beers law graph slope of 0.22L/mg of IronII and a y intercept I need to find the percent of the Iron II ion in the sample.

Homework Equations

I've never heard of beers law before...I think this is the equation, but I have no clue how to use it, or which one from here to use.

http://en.wikipedia.org/wiki/Beer–Lambert_law


How does this law work, and how can I go about doing this calculation?

 

[Borek]

Basically concentration is directly proportional to absorbance, that's all you need.

c = intercept + k*absorbance

In theory intercept should be 0, in practice it is not always the case.

 

[Lancelot59]

So C is the concentration, the intercept is the absorbance when the concentration is zero, and absorbance is...the absorbance of the unknown. Right.

Then is K the slope?

 

[Borek]

Yes. It should be absorbance coefficient times path length, but when doing experiment we usually prepare calibration curve and we don't pay attention to details, we are just interested in slope, as that's enough.

 

[Lancelot59]

Path length? Is that the distance it travels through the sample? Also how does the mass of the sample factor into this? I'm sure we were given it for a reason.

 

[Borek]

Yes for path length.

What is definition of percentage?

 

[Lancelot59]

It's the fraction of something out of 100 parts...

Sorry, I'm not really understanding this. :/ If I have the mass and the absorbance average, then I can plug it into get this:

concentration of ion in sample = (slope as L/mg of ion)*absorbance(average) + intercept

So how does the mass of the whole sample factor into this?

 

[Borek]

Now, what is the definition of concentration?

I am guessing how the experiment looked alike, so I can be wrong. Please outline what you did.

 

[Lancelot59]

Well, it's not an experiment. It's a just a question we need to do given that data.

Concentration is mols per unit of volume...but how does that help? I don't know what percentage of the mass of the sample is the iron, or water.

Could I divide the slope by the molar mass of iron to get the mols? It is in L/mg.

 

[Borek]

Do you know volume of the solution? Can you calculate amount of iron?

 

[Lancelot59]

No, I don't know the volume.

 

[Borek]

So you can't solve - could be that's the information you forgot to ask for (in case anyone reads it later - we started discussion on chat).

 

[Lancelot59]

I actually just had a little breakthrough.

The information I have:

slope = 0.2200 L/mg Fe2+
intercept = .004

and the info I that lost was the sample mass and the absorbance of the sample.

So in y=mx+b form the setup goes to:

absorbance=(0.2200)(concentration of ion in mg/L)+(.004)

so the absorbance I lost on the way home would go into that, then I can isolate the concentration...and then what?

I need to find the mass of the Iron. I know the total mass (or rather did till the sheet got trashed), but I don't know the volume...OR DO I!? It says it follows the same procedure as outlined in the lab manual. Give me a little bit, I think all this random bouncing of ideas just paid off. I can just get the volume of the sample from there. After that I can multiply the concentration by that and get the mass.