Lab 4 Post Lab

# Calculate the error and discuss abs409kjmol

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Calculate the % error and discuss. abs(40.9kJ/mol-34.89kJ/mol)/34.89kJ/mol X 100% = 17.23% My value for enthalpy is not too far off. This means we preformed the experiment pretty accurately, within 20%.Some errors that could have occured are that we had to redo the first crystallization and when the solution was heating some of it bubble out which could give inaccurate numbers. 4. The literature value for the entropy of solution of KNO3 is 248 J/mole.K. How does your value compare? Calculate the % error and discuss. abs(159.91J/Kmole-248J/Kmole)/248J/Kmole X 100% = 35.52% This percent is a little higher than the percent error for enthalpy and the errors could be the same as the ones stated above or could be a number of other ones. In addition, there could have been a problem with the set up or the materials provided in the lab. PURPOSE AND METHOD Part I The purpose of part one of this experiment is to determine if crystallization and recrystallization improve the purity of a KNO3 solution, and if it does in fact improve it, to determine how much. The solubilities of KNO3 and (NH4)2Fe(SO4)2 6H2O and significantly different, KNO3 will crystallize at 0 degrees Celsius which is before (NH4)2Fe(SO4)2 6H2O will crystallize (it remains a liquid at this temperature). This causes the two substances to separate. We were able to take more of the impurity of KNO3 out by repeating the process. We then added 1, 10-phenanthroline to the solution, which using Beer’s Law allowed us to determine the concentration: Absorbance = absorptivity x 1 (1 cm x 1 cm cuvette) x concentration. The mass of the impurity can be calculated by: M = moles/L (solve for moles then convert moles to grams). The percent impurity of KNO3 can be determined by: (mass of (NH4)2Fe(SO4)2 6H2O / mass of crystallized product). Part II In part two of the lab we created a solubility curve for KNO3. We collected six data points for temperature data where crystallization of different concentrations of KNO3 was occurring. We solved for the Ksp values and then plotted ln(Ksp) vs. 1/T. we calculated the delta H of the solution and the delta S of the solution by using the equation ln(Ksp) = (-deltaH/R)(1/T) + (deltaS/R) where deltaH = -(m(slope)*R) / 1000 and deltaS=(b(y-int)*R). Does the absorbance exceed the desired range (0 to 2.0 A)? ______yes______ If so, what dilution factor did you use in order to get the absorbance in the desired range? _____1:10________ Note: Data is being restated here for ease in plotting. For the mass of KNO3 crystallized in crystallization #0, the starting mass of KNO3 you weighed out in Step A.3. will be the mass crystallized, resulting in 100% recovery. The cells for the mass of KNO3 crystallized will autofill from cells G53, G64, and G75. The % of original KNO3 mass will autofill from G65 and G76. Plot a graph of solubility of KNO3 (g KNO3 / 100 g H2O) vs. Temperature(oC) and place it here. Cover this instruction box so that you graph is appropriately sized.
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