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Spectrochemical Analysis of Copper

Name:___________________________________ Section: ___________

Date: ________________

DATA

Mass of CuSO4·5H2O used to prepare standard Cu2+ solution: ___________

Concentration of standard Cu2+ solution prepared: _____________

Wavelength verses percent transmittance and absorbance:

nm |400 |420 |440 |460 |480 |500 |520 |540 |560 |580 |600 | |%T | | | | | | | | | | | | |A | | | | | | | | | | | | |

nm | | | | | | | | | | | | |%T | | | | | | | | | | | | |A | | | | | | | | | | | | |

Determination of λmax: Attach a graph (use x-y scatter in Excel) of absorbance (on the y-axis) vs. wavelength (on the x-axis). Label the wavelength that corresponds to maximum absorption (λmax).

Wavelength for maximum absorption (λmax): ________ nm

Calibration Curve and Unknown: Unknown # _________

Tube No. |1 |2 |3 |4 |5 |6 |7 |Unk. | |Vol. Cu2+ soln. (mL) | | | | | | | | | |Vol. “en” reagent (mL) | | | | | | | | | |Vol. distilled water (mL) | | | | | | | | | |New Cu2+ conc. ( ) | | | | | | | | | |% T | | | | | | | | | |A | | | | | | | | | |Show calculations for new Cu2+ concentrations in the table above (include units):

Calibration curve: Attach a graph of the data (use x-y scatter in Excel) of absorbance (on the y-axis) vs. concentration of Cu2+ (on the x-axis). Include a trendline (best linear-fit) through the data. Show both the equation and the R2 value for this line on the graph.

Determination of Unknown Concentration: Using the best-fit equation from the calibration curve calculate the concentration of Cu2+ in the unknown sample after dilution (with “en” and distilled water).

Concentration after dilution: ________________ (give units)

Show the best-fit equation and calculations:

What was the concentration of your unknown before dilution to 7 mL? _____________

Show your calculations here:

Post Lab Questions

1. Absorbance depends on the thickness of the sample size. The cuvettes for most spectrophotometers hold the sample size (“b”) to 1 cm, although they can vary. The molar absorptivity (“a”) is equal to the absorbance of a one-centimeter thick sample of a 1.0 M solution. In this case, “a” has the units L/mol·cm. Absorptivity changes with wavelength and is generally given for the wavelengths corresponding to the maxima in the absorption curve. How is the slope of the calibration curve related to the molar absorptivity?

2. A solution known to contain 0.026 M Cu2+ forms a complex with ethylenediamine that yields an absorbance of 0.37 in a cell of 1.0 cm path length at a selected wavelength. Calculate the molar absorptivity for these conditions.

3. Calculate the concentration of an unknown copper solution with an absorbance of 0.49 under the same conditions given in question 2.

4. From the absorption curve of Cu(en)22+ you constructed in the first analysis (A versus wavelength) fill in the following (show work or give your reasoning).

• The molar absorptivity, a, at the maximum wavelength ________ L/mol·cm

• The molar absorptivity at 50 nm past the

maximum wavelength (toward longer wavelength) ________ L/mol·cm

• The molar absorptivity at 50 nm before the

maximum wavelength (toward shorter wavelength) ________ L/mol·cm

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