# Molar absorptivity and concentration relationship

### Beer's Law Tutorial Law is the linear relationship between absorbance and concentration of an Where is the wavelength-dependent molar absorptivity coefficient with units of. When working in concentration units of molarity, the Beer-Lambert law is written as: A = * b * c. where is the wavelength-dependent molar absorptivity coefficient with units of M-1 cm Data are The relation between A and T is: A = -log T. The relationship between absorbance and transmittance is illustrated in the following diagram: So, if all e is the molar absorbtivity with units of L mol-1 cm-1 b is the c is the concentration of the compound in solution, expressed in mol L- 1.

## The Beer-Lambert Law

The Greek letter epsilon in these equations is called the molar absorptivity - or sometimes the molar absorption coefficient. The larger the molar absorptivity, the more probable the electronic transition.

In uv spectroscopy, the concentration of the sample solution is measured in molL-1 and the length of the light path in cm. Thus, given that absorbance is unitless, the units of molar absorptivity are L mol-1 cm However, since the units of molar absorptivity is always the above, it is customarily reported without units.

Guanosine Guanosine has a maximum absorbance of nm. What is the concentration of guanosine?

### Chem - Experiment II

What is the extinction coefficient? Suppose you have got a strongly colored organic dye. If it is in a reasonably concentrated solution, it will have a very high absorbance because there are lots of molecules to interact with the light. However, in an incredibly dilute solution, it may be very difficult to see that it is colored at all. The absorbance is going to be very low.

Suppose then that you wanted to compare this dye with a different compound. Unless you took care to make allowance for the concentration, you couldn't make any sensible comparisons about which one absorbed the most light. The absorbance is not likely to be very high. On the other hand, suppose you passed the light through a tube cm long containing the same solution.

More light would be absorbed because it interacts with more molecules. Is the slope of the calibration line at nm greater than, less than, or equal to the slope at nm? You can choose any wavelength to create a calibration plot, the only differerence will be the slope of the line. When you actually choose your wavelength to create your calibration graph, you would generally like to choose a wavelength where there is room for the concentration to decrease.

## Beer-Lambert Law

Look at the spectrum above. Do you think nm would be a good wavelength to use for a calibration graph? You would not choose that wavelength because when you lower the concentration, you would not be able to see much of a difference in the absorbance, and the calculations would be inaccurate. You would most likely want to choose wavelengths like nm or nm where there is a lot of room for absorbance change.

Now for the fun part! Using the calibration plot that YOU made from the data two pages ago.

### Beer's Law - Theoretical Principles

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.