Lab Report

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LabReport

Title:Spectrophotometric Determination of Equilibrium Constant

Objectives

Theexperiment aimed at establishing the equilibrium constant (Kc)for the equation:

Fe+3+ SCNFeSCN+2

Materials

  • 5- 100mL Volumetric Flask

  • 0.2M Fe(NO3)3

  • 0.002M KSCN

  • 0.1M HNO3

  • Spectrophotometer

  • Cuvettes

  • Five capped vials

Outlineof Procedure

  • Five standards were prepared using the 100mL volumetric flasks labeled as solution 1-5 as below:

Solution

0.2M Fe(NO3)3 in 0.1M HNO3 (in mL)

0.002M KSCN in 0.1M HNO3

0.1M HNO3

1

25.0

0.0

75.0

2

25.0

3.0

72.0

3

25.0

6.0

69.0

4

25.0

9.0

66.0

5

25.0

12.0

63.0

Thesolutions were allowed to mix thoroughly.

  • A spectrometer was connected to the computer and the Open Logger Pro software used in recording the observations.

  • Several portions of solution one were used to rinse the cuvette, and this was labeled as a blank solution.

  • After setting up the spectrometer, it was configured by selecting absorbance vs. concentration, and the name changed to [FeSCN+2). The wavelength was also set at the particular scale.

  • Using the dilution equation (M1VI = M2V2), the concentration of [FeSCN+2] was calculated. Each of the solutions was placed in a cuvette and absorbance read.

  • Next, five other solutions were prepared to determine their equilibrium constants in capped vials as shown below:

    Solution

    0.2M Fe(NO3)3 in 0.1M HNO3 (in mL)

    0.002M KSCN in 0.1M HNO3

    0.1M HNO3

    1

    5.0

    1.0

    4.0

    2

    5.0

    2.0

    3.0

    3

    5.0

    3.0

    2.0

    4

    5.0

    4.0

    1.0

    5

    5.0

    5.0

  • Using the spectrometer, the readings of each was recorded.

Data

Thetable below shows the data obtained

  1. For the equation Fe+3 + SCN FeSCN+2, the equilibrium constants were as shown:

Solution

[FeSCN+2]eq

1

5.7*10-5

2

8.6*10-5

3

1.3*10-4

4

1.8*10-4

5

1.9*10-4

Theequilibrium constant for the reaction as shown below:

Solutions at equilibrium

[Fe+3]

[SCN]

[FeSCN+2]

1

9.43*10-4

1.43*10-4

5.7*10-5

2

9.14*10-4

3.14*10-4

8.6*10-5

3

8.7*10-4

4.7*10-4

1.3*10-4

4

8.2*10-4

6.2*10-4

1.8*10-4

5

8.1*10-4

8.1*10-4

1.9*10-4

Calculations

TheKcforeach of the determinations was as shown below using the formula

Solution

Kc

1

4.23*102

2

3.0*102

3

3.18*102

4

3.54*102

5

2.90*102

Average Kc

3.37*102

Conclusion

Determiningthe equilibrium constant of a reaction can be easily done using aspectrophotometer. With set guidelines and precision especially incalibration, the equilibrium constant can be understood.Additionally, the spectrophotometer allows generation of graphs toshow the calibration curve of the absorbance against molarconcentration.