Calculate the concentration of an unknown copper solution with an absorbance of 0.49 under the same conditions.

Spectrochemical Analysis of Copper
 
 
Name:___________________________________ Section: ___________
 
 
Partner:__________________________________ Date: ______________
Determination of max
 
Complete the following tables of absorbance by wavelength:
 

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

 
 

nm                  
A                  

 
 
 
Create a graph (use smooth-marked x-y scatter in Excel) of absorbance (on the y-axis) vs. wavelength (on the x-axis). Include ALL of the above data points. Label the wavelength that corresponds to maximum absorption (max) on your graph. Attach a copy of the graph.
 
Wavelength for maximum absorption (max): ___________
 
 
The Standard Curve Concentration of standard Cu2+ solution: ___________
 
Unknown#: ___________
 
 
Complete the following table with data from your standard solutions and the unknown dilution.

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. (M)                
Average Absorbance                

 
 
Show all the calculations for Tube 7. Be sure to include calculations for both total volume and concentration.
 
 
 
 
 
 
 
 
 
 
 
 
 
Create a calibration curve using your standard solution data. In Excel, graph absorbance (on the y-axis) vs. concentration of Cu2+ (on the x-axis) using the marked x-y scatter. Include a trendline (best linear-fit) through the data. Show both the equation and the R2 value for this line on the graph. Attach a copy of the graph.
 
Calibration curve best-fit equation: ____________________________
 
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). Then determine the concentration of the unknown before it was diluted and the percent error of your result.
 

Conc. after dilution (M) Conc. before dilution (M) Actual Conc. (M) % Error
       

 
 
Show the calculations:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Preparation for Writing a Formal Report
 
The discussion section of a formal report evaluates the data and observations for the experiment (i.e. the results). TYPE a discussion section for the Spectrochemical Analysis of Copper experiment and attach it to this form. This section should interpret the results in context of the purpose and scientific theory (as presented in the introduction). Both accuracy and precision should also be evaluated. Discuss sources or error and how the experiment could be improved.
 
 
 
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.
a. Calculate the molar absorptivity for these conditions.
 
 
 
 
 
 
b. Calculate the concentration of an unknown copper solution with an absorbance of 0.49 under the same conditions.

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