Tutorial for Virtual Laboratory on

Beer's Law Plot

Part 1; Tips for Running the Experiment:

 

OK, you have no instructions. All student's have the same compound but different concentrations. That is, we use Beer's Law for Quantitative Analysis not Qualitative Analysis. You will run all measurements at 650 nm and so you only need to "blank" out the solvent once. Now, in principle, you are measuring the amount of light absorbed by a series of solutions of known concentration and these you use to create your Beer's Law Plot. Then, after you make your Beer's Law Plot, you measure the absorbance of your unknown and compare that to your knowns (the Beer's Law Plot).

 

Note, absorbance measurements are very sensitive and your knowns are very dilute with concentrations of the order of micro molar. If your unknown is more concentrated than the known it may absorb too much light for the spectrometer to measure. In this case the absorbance will be off scale and you need to dilute your unknown until it is of an appropriate concentration to measure with the spec 20.

 

 

Part 2; Tips on Operating the Virtual Lab:

 

In the first part you are going to make a series of half dilutions of the known solution

( 6.5micro molar S5I). You can transfer a 250 ml Erlenmeyer flask to the work bench and add 50 mL of water. Then right click on it (3 times) and choose "Duplicate". Now right click on each of these flasks and choose "Rename" and label the 2-4. Now simply transfer the amount of stock you wish to dilute to the first one (flask 2) and sequentially dilute your known solution. Remember the stock solution is one of your data points

 

Tips for diluting Unknown: We live in a base 10 decimal world, so use a dilution factor of 1:10. That is, each time you make a 10-fold dilution you simply move the decimal one place. So 4 10-fold dilutions is 1/10,000 the concentration of the original. Alas, you dilute your unknown down to an absorbance where the Spec 20 works, calculate the concentration of your diluted unknown, and then back calculate the concentration before dilution.

 

Part 3; Comparison of Virtual Lab to Real Lab:

 

In the Virtual Lab the volumes of solutions are additive. That is 8 mL of one solution mixed with 8 mL of another solution are 16 ml. In the real lab this is seldom the case.

 

The virtual spec 20 actually displays a spectra while the real one does not.