Experiment 34 - exp 43 lab report PDF

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Experiment 34: An Equilibrium Constant

Data: Table 1: Measurements Used in the Experimental Setup

Standard Solutions Volume of NaSCN (mL) Moles of SCN(mol)

Molar concentration of Fe(NO3)3 (M)

0.2

Molar concentration of NaSCN (M)

0.001

Blank

1

2

3

4

5

0

1

2

3

4

5

0

1.00*10-6

2.00*10-6

3.00*10-6

4.00*10-6

5.00*10-6

[SCN-] (25.0 mL) [FeNCS2+] (mol/L) %T Absorbance

0

4.00*10-5

8.00*10-5

1.2*10-4

1.6*10-4

2.00*10-4

0

4.00*10-5

8.00*10-5

1.2*10-4

1.6*10-4

2.00*10-4

100

12.45

63.10

41.88

34.75

21.04

0

.095

.200

.378

.459

.677

A vs. [FeNCS2+] 0.8 0.7 0.6

f(x) = 3325 x − 0.03 R² = 0.98

0.5 0.4 0.3 0.2 0.1 0

0

0

0

0

0

0

Table 2: Absorbance for the Set of Test Solutions

Test Solutions Volume of Fe(NO3)3 (mL) Moles of Fe3+, initial (mol) Volume of NaSCN (mL)

Molar concentration of Fe(NO3)3 (M)

0.002

Molar concentration of NaSCN (M) 6 7 8 5 5 5

0.002 10 5

9 5

1*10-5

1*10-5

1*10-5

1*10-5

1*10-5

1

2

3

4

5

Moles of SCN-, initial (mol)

2*10-6

4*10-6

6*10-6

8*10-6

1*10-5

%T Absorbance

84.92 .071

56.89 .245

31.84 .497

19.36 .713

12.11 .917

Table 3: Calculations of Kc [FeNCS2+], equilibrium, from calibration curve (mol/L) Moles FeNCS2+ at equilibrium (10 mL) (mol) [Fe3+], equilibrium Moles Fe3+, reacted (mol) Moles Fe3+, equilibrium (mol) [Fe3+], equilibrium (unreacted) (10 mL) (mol/L) [SCN-], equilibrium Moles SCN-, reacted (mol) Moles SCN-, equilibrium (mol) [SCN-], equilibrium (unreacted) (10 mL) (mol/L)

FeNCS ¿ 2+¿ ¿¿ ¿ Fe ¿ 3+¿ SCN ¿ −¿ ¿ ¿ ¿ ¿ K c=¿ Average Kc

6 2.15*10-4 2.15*10-6

2.15*10-6 7.85*10-6 7.85*10-4

2.15*10-6 1.5*10-7 1.5*10-5 1.8*104

3*103

7 2.47*10-4 2.47*10-6

2.47*10-6 7.53*10-6 7.53*10-4

2.47*10-6 1.53*10-6 1.53*10-4 2*103

8 2.79*10-

9 3.46*10-

4

4

2.79*10-

3.46*10-

6

6

2.79*10- 3.46*106

6

7.21*10-

6.54*10-

6

6

7.21*10- 6.54*104

4

2.79*10-

3.46*10-

6

6

3.21*10-

4.54*10-

6

6

3.21*10- 4.54*104

4

12.05

11.65

10 3.88*10-4 3.88*10-6

3.88*10-6 6.12*10-6 6.12*10-4

3.88*10-6 6.12*10-6 6.12*10-4 10.36

Discussion:

Looking at the aim of the experiment, the aim was accomplished. The data from this lab shows that as concentration of a solute such as FeNCS2+ increases, the absorbance will also increase. The experiment was performed in two parts: part one was done to establish a standardization curve, which yielded the slope equation, and part two was done to calculate the moles of Fe3+ and SCN-. After calculating the absorbance values and the concentration of FeNCS2+ for each trial, a graph was made comparing the concentration of FeNCS2+ to the absorbance values, with the slope of the linear relationship used to calculate the molar concentration of FeNCS2+ equilibrium. In this lab, the k constant was also determined; it came out to be 3*103. This number is also known as the equilibrium constant. The data is pretty reliable because of the strong positive correlation number. This means that the line of best-fit equation y = 3325x +.031 is very reliable in extrapolating values. The line also shows how the values are following a trend in which as the concentration of FeNCS2+ increases, so does the absorbance. During the experiment there could have been a possible error of not cleaning the cuvet completely before placing it in the spectrophotometer. This would have given inaccurate percent transmittance values. Conclusion: The overall purpose of the lab was to use a spectrophotometer to determine the equilibrium constant of a chemical system and determine the equilibrium constant for a soluble equilibrium. The absorbance values increased consistently throughout the trials, so this shows that the data was accurate. The overall average Kc was found to be 3 x 103.

Post Lab Questions:

3.) a. The equilibrium concentrations of FeNCS2+ in the Test Solutions (Part B) will be too high because the absorbance used for the graph would be lower, leading to a decrease in slope. b. The calculated Kc for the equilibrium will be too high by the erred data plot because the concentration of product, FeNCS2+, is higher and concentration of reactants, Fe3+, and SCN-, is lower. 4.) a. This technique error will cause the absorbance reading for FeNCS2+ in the analysis to be too low because less light will go through the cuvet (due to the fingerprint smudges), thus, the absorbance reading will be lower. b. The equilibrium concentration of FeNCS2+ will be recorded as being too high because the absorbance will be read as being low. c. The equilibrium concentration of SCN- will be too low because the concentration of FeNCS2+ will be higher. d. The Kc for the equilibrium will be too high because concentration of FeNCS2+ is larger and the concentration reactants, Fe3+ and SCN-, are lower (decreased).

6.) The sample can still be used for analysis by diluting the sample to decrease the concentration, which leads to an increase in absorbance.

Reference: 1

Be r a n, J . A. ,Labor at or yManualf orPr i nc i pl e sofGe ne r alChe mi s t r y ,Te xa sA&M Uni v e r s i t y , t h 10 e di t i on:2014, pp377388....