Lab - Equilibrium Constant Using a Spectrophotometer
Uploaded by erobinson on Apr 30, 2004
Determination of an Equilibrium Constant Using a Spectrophotometer
Introduction:
In this lab we will be given a variety of different solutions with known amounts of each substance. We will be determining the concentration of Fe(SCN)2+ by using a spectrophotometer to find absorption. We will finally find K for each solution by finding the initial and final concentrations and calculating.
Data:
[img:3171928e69]http://www.collegepimp.com/echeat/lab14.gif[/img:3171928e69]
Discussion:
This experiment was one of the faster ones thus far. The solutions were already prepared for us, I assume so that everyone would have similar results and that there would be less of a chance of error (although we did find a major one). All that was required of us for data collection was calibrating the spectrophotometer, and then placing cuvettes of solution into the spectrophotometer and reading the absorption. The tricky part was calculations.
The main point of this lab appears to be to prove that the K constant is just that, constant. We found the K constant to be an average of 288 with a standard deviation of 10.4. The actual K constant is not given to us so it is unknown whether this was an accurate result, but the fact that there was only a deviation of 10.4 from mean shows that if the solutions and spectrophotometer were perfect, then our K values would be the same for each solution.
The way we found K for each solution was to find the initial concentrations of Fe3+, SCN-, and Fe(SCN)2+, then to determine x by using the equation for the trend line on our calibration graph. From there we found the final concentration or equilibrium concentration for each reactant and product by either subtracting or adding as appropriate. Finally we plugged the numbers to the equation like so: K = [Fe(SCN)2+ + x] / ([Fe3+ - x] * [SCN- - x]).
We found a major error in solution A-7 and B-5. Both of these solutions seemed to be identical to A-6 and B-5 respectively. The only source would be a human error of accidentally making solutions A-6 and B-5 again. The fact that A-7 was incorrect most likely skewed the calibration graph slightly, but we chose to ignore B-5 when calculating the average K value so that it would not alter our numbers.