# Path length and absorbance relationship goals

### Spectrophotometry example (video) | Kinetics | Khan Academy

Your goal is to conduct an experiment to determine the concentration (in mM) you have values for absorbance, path length, and the extinction coefficient, you. Thus, if the path length and the molar absorptivity are known and the absorbance is . As suggested by the above relationships, the absorbance scale is the most useful for colorimetric assays. Using a .. The goal of these gels is to maximize. By measuring the Absorption Spectrum of a substance, i.e., all the For general purpose work, we utilize broad range bulbs which allow absorbance to be law describes an important relationship that exists between absorbance (A) and two sample parameters - solute concentration (c) and length of the light path (l).

This is the currently selected item. Video transcript Let's see if we can tackle this spectrophotometry example. So let's see what the problem is. It says a solution of potassium permanganate-- let me underline that in a darker color-- potassium permanganate has an absorbance of 0. So this nanometers is the wavelength of light that we're measuring the absorbance of.

And so this is probably a special wavelength of light for potassium permanganate, one that it tends to be good at absorbing. So it'll be pretty sensitive to how much solute we have in the solution. OK, and the beaker is 1 centimeter.

So that's just the length right there. What is the concentration of potassium permanganate? Prior to determining the absorbance for the unknown solution, the following calibration data were collected for the spectrophotometer. The absorbances of these known concentrations were already measured. So what we're going to do is we're going to plot these.

And then, essentially, this absorbance is going to sit on the line. We learned from the Beer-Lambert law, that is a linear relationship between absorbance and concentration. So this absorbance is going to sit some place on this line.

### Chem - Experiment II

And we're just going to have to read off where that concentration is. And that will be our unknown concentration. So let's plot this first. Let's plot our concentrations first. So this axis, the horizontal axis, will be our concentration axis. I'll draw the axis in blue right there. Let me scroll down a little bit more. I just need to make sure I have all this data here. So this is concentration in molarity. And let's see, it goes from 0. So let's make this 0. This over here is 0. One, two, three, then this over here is 0.

And then this over here is 0. And then the absorbances go-- well it's close to 0, or close to 0. So let's make this right here 0. Let's make this 0. And that, essentially, covers all of the values of absorbency that we have here. So let's plot the first one. When we had a concentration of potassium permanganate at 0. And then when we had 0.

## Spectrophotometry example

And we already see an interesting line form, but I'll plot all of these points. And then at 0. So this is 0.

This would be 0. And actually, what we're doing here, we're actually showing you that the Beer-Lambert law is true. At specific concentrations, we've measured the absorbance and you see that it's a linear relationship. Anyway, let's do this last one. So this right here is 0. I want to make sure I don't lose track of that line. So you see the linear relationship?

Let me draw the line. I don't have a line tool here, so I'm just going to try to freehand it. I'll draw a dotted line. Dotted lines are a little bit easier to adjust. I'm doing it in a slight green color, but I think you see this linear relationship. This is the Beer-Lambert law in effect. Now let's go back to our problem. We know that a solution, some mystery solution, has an absorbance of 0. I'll do it in pink-- of 0. So our absorbance is 0. Now for the fun part!

Using the calibration plot that YOU made from the data two pages ago. We are going to determing the concentration of an unknown solution. Make sure you have your plot ready, because here we go! Here's a typical problem. You take 3mL of your unknown sample and 7mL water and mix them together. The dilluted sample gives an absorbance of 0. What is the concentration of the initial unknown?

Where do you begin?! You have an absorbance, and you have a straight line equation that relates absorbance to concentration.

This is the line of best fit through your data. Remember you dilluted it once, so you can use the Dilution Equation Ready to try one on your own? Here are a few more problems.