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Equilibrium Constant

Bettina Mae De Mesa

3/20/2014

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Abstract

The main purpose of this lab was to determine the equilibrium constant within a chemical reaction with the utilization of a spectrophometer. With the use of graphing techniques as well as data analysis, the equilibrium constant for a soluble equilibrium can be determined. The reference solutions are concocted by mixing an excess of Fe3+ ions with the known compounds of the SCN- ions. The assumption is that the reactions are going to be driven to completion due to Le Chatelier’s principle, knowing that they contain a known concentration of FeSCN2+ ions. Secondly, the various test solutions are made by mixing together a constant amount of the Fe3+ ions with varying amounts of the SCN- ions, which ultimately contain the unknown concentration of FeSCN2+ ions. The absorbance of the solutions are then measured with a spectrophomter. Referencing back of the solution’s recorded absorbance, a calibration curve is made then to determine the concentration of each of the test solutions. Ultimately, the calculated concentrations can be utilized in order to determine the equilibrium constant.

In terms of results, the average equilibrium constant was 287.0. Referencing back to lecture, Professor Miles stated that the results should be close to 250.0. With the use of that observed and expected, the percentage error formula was used and ultimately obtained a finding of 14.80% error. This percentage error is relatively small, asserting my confidence in the outcome of my results. Standard deviation is the measure of how precise the average is, leading the calculation of the standard deviation of Kc to be 39.5%. This high number can be linked back to random error. The relative standard deviation on the other hand measures the precision, not accuracy of the results. My findings came out to be 13.76% RSD.

Introduction

The use of a spectrophometer method of analysis includes the

Experimental Methods

In this experiment, a set of standard solutions with known concentrations were used in order to determine the A for each solution. A spectophometer was used in order measure the transmitted light intensities, and ultimately measure the absorbance on each substance. A blank was used in

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order to set the guideline for the measured graph. With the use of data studio, automatic recording software, the absorbance of each test solution was monitored and hand recorded. The Microsoft Excel document was also used in order to create the absorbance versus concentration curve in part A. The programmed logarithms produced a scatter plot displaying the slope and r2 value of the graph. Using the slope, the equilibrium of the reaction was determined. Lastly, in order to verify the given results. The Citrus College verification software was used in order to assert the notion that the data collected is indeed correct. Extra calculations specified by the professor included the relative standard deviation as well as the percentage error. Both formulas can be found in the Beran lab manual and manually calculated.

In terms of experimental procedure, the volume of test solutions needed were followed exactly to protocol. The clean up however included a thorough washing of the 25mL flasks as to avoid the staining of the glassware itself.

Data

0 0.00005 0.0001 0.00015 0.0002 0.000250

0.1

0.2

0.3

0.4

0.5

0.6

f(x) = 2586.76470588235 xR² = 0.999511778951944

Absorbance Vs. FeNCS2+

[FeNCS2+]

Abso

rban

ce

Caption: In this linear graph, the absorbance value is directly proportional to the molar concentration of the utilized substance. With the use of a calculated slope as well as the corresponding absorbance data, the molar concentration of the same absorbing species in a solution of unknown concentration can be found. The slope of this particular graph is 2586.8 while the r2 value is getting closed to 1.

Discussion

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Conclusion

In this lab, a series of steps were conducted in order to establish the equilibrium constant of [Fe(NCS)]2+. With the use of the calculated concentration equilibrium for each solution, a calibration plot involving absorbance versues the concentration of [Fe(NCS)]2+ was made. From this graph, it was determined that the concentration of the