Sabrina Fregoso
Professor Farnum
Chem 112
MW 7:35 am
27 September 2018
Experiment 34: An Equilibrium Constant
Abstract:
In this experiment, the equilibrium constant of a chemical system was determined using a
spectrophotometer. This process was done using a spectrophotometer connected to the Pasco
Capstone software to determine the absorbance and percent transmittance. First, a set of standard
solutions were prepared and calibrated through a spectrophotometer to establish a calibration
curve from the absorbance and percent transmittance. Then that same process was followed to
get the absorbance for a set of test solutions. The data collected in the first part was plotted using
Microsoft Excel. The equilibrium constant for a soluble equilibrium was determined through
calculations using the slope from the calibration curve. As a result, the average magnitude of an
equilibrium constant, K
c
, was ~ 309.36. The standard deviation of K
c
was ~55.025. The relative
standard deviation of K
c
was ~17.79%RSD.
The quality of the data taken through the Pasco
Capstone software was executed well and the graphs that were as a result, were neat and precise.
Introduction:
The main objective of this experiment is to determine the equilibrium constant, K
c
, of a
set of test solutions. This objective will be met by conducting a series of chemical mixtures and
gathering the absorbance and transmittance using a spectrophotometer connected to the Pasco
Capstone software. The Pasco software constantly records the absorbance and percent
transmittance simultaneously. There is a total of five test solutions that have 5 mL of 0.002 M
Fe(NO
3
)
3
, different volumes (mL) of 0.002 M NaSCN, and different volumes (mL) of 0.1 M
HNO
3
. The overall reaction is expressed as:
Equation 1:
Fe
3+
(aq)
+ SCN
-
(aq)
↔ FeSCN
2+
(aq)
It is assumed that the position of the equilibrium is driven so far to the right due to the excess
Fe
3+
, this as a result complexes all the SCN
-
to form FeSCN
2+
.
Spectrophotometric methods involve the interaction of electromagnetic (EM) radiation
with matter. The most common EM spectrums used for analysis are the ultraviolet, visible, and
infrared regions. The degree of absorbed radiation or the intensity of the transmitted radiation is
measured using a spectrophotometer, which measures transmitted light intensities with
photosensitive detector at a specific visible wavelength. The wavelength that the absorbing ions
or molecules have a maximum absorption of visible radiation is determined and set in the
