Experiment 2:  Determination of the Equilibrium Constant
for the Reaction
After rinsing the cuvette three times and then filling it with your solution, wipe off the outside of the cuvette with the tissues provided in the box next to the spectrophotometer. (This will remove any moisture or dirt on the cuvette that would give a false reading).  Take the blank out of the machine and insert your cuvette very carefully making sure that the brand marking on the cuvette points toward you.  Insert the cuvette into the holder until it stops, then close the lid.
Do not touch any of the knobs, just take the reading on the machine as the absorbance of your solution.  The absorbance reading of this particular solution is 1.100 (absorbance does not have any units).

Now, take the absorbance reading you read from the spectrophotometer and use it to read concentration of FeSCN2+ off the graph of absorbance vs. concentration in your lab handout.  In our example we will find our absorbance reading, 1.100 on the absorbance axis (y-axis of the graph), scan across the graph horizontally until you reach the diagonal line, then scan straight down to the concentration axis (x-axis of the graph).  (The red line is a guide for the eye only).  In our example, the concentration is read as 34.4  X  10-5 mol/L.  In reading the graph correctly, one must be aware of the details of the graph.  First, the distance between large increments on the absorbance axis is 0.2.  Since there are 10 small increments between each of the large increments, what do you think of the size of the smallest increments is?

Likewise, the distance between large increments on the concentration axis is 0.4 mol/L.  Since there are 10 small increments between the large markings, what do you think the size of the small markings are?
 
Our concentration reading fell six small increments to the right of the 32 mark, so our final reading from the axis is 32 plus 6 small increments is 34.4.  Note that all measurements for the concentration axis must be multiplied by 10-5 mol/L to be correct.  If your concentration is missing this last detail, your calculations will be very incorrect.