Beer's Law Lab: Determination of the Unknown Concentration of Cuso4

Beer's Law Lab: Determination of the Unknown Concentration

of CuSO4

Scott Green 9-29-14 Purpose The purpose of this lab was to prepare and test the absorbance of five standard copper (II) sulfate solutions. Using these absorbance's graph and calculate a standard curve to calculate the absorbance of an unknown molar concentration copper (II) sulfate solution. Part 2 of the lab was to calculate and make 20 mL of a 0.40 M solution of copper(II) sulfate and determine its % absorbance.

Procedure Part 1

1. Obtain a small volume of 0.40 M CuSO4 solution and distilled water in separate beakers. 2. Label four clean, dry, test tubes 1–4. Use pipets to prepare five standard solutions according to the chart below.

Trial 0.40 M CuSO4 Distilled H2O Concentration number (mL) (mL) (M) 1 2 8 0.080 2 4 6 0.16 3 6 4 0.24 4 8 2 0.32 5 ~10 0 0.40

3. Using a Colorimeter and a Lab Quest determine the percent absorbance's for each solution of

CuSO4 4. Obtain an unknown solution and determine its percent absorbance 5. Use Logger Pro to determine the slope of the line (y=mx+b) and calculate the molar concentration of the unknown.

Part 2 1. Calculate grams needed to make 20 mL of 0.40 M solution of copper II sulfate pentahydrate 2. Determine the percent absorbance of the solution made using a Colorimeter and a Lab Quest .

Experimental Data Part 1

Trial Concentration (mol/L) Absorbance 1 0.080 0.042 2 0.16 0.176 3 0.24 0.303 4 0.32 0.452 5 0.40 0.558 6 Unknown number ____ Best Fit line Equation ABS = 1.635 x + -0.08620

Absorbance of unknown _____

Part 2 Absorbance of student made solution of 20 mL of 0.40 M solution of CuSO4∙ 5 H2O ______Calculations and Graph Part 1

Molarity of unknown: 0.233 = 1.635 x + -0.08620 x = 0.177

Part 2 Given Calculations

0.40 M 0.40 M x 0.020 L = 0.008 moles CuSO4∙ 5 H2O 20.0 mL 0.008 moles CuSO4∙ 5 H2O x = 1.99 g Absorbance 0.819 Conclusion By utilizing the linear relationship between concentration and absorbance known as Beer’s law, it was determined that the concentration of the unknown solution was 0.177M. The second part of the lab the absorbance was determined to be 0.819.

Theory/ Purpose of the lab The theory used in this lab is that there is direct relationship between absorbency and concentration for a solution. As the number of ions increase the absorbance should decrease. This was used to determine the molar concentration of an unknown. It is also used to determine the accuracy of a sample prepared given molarity and volume.

Error analysis The % error for lab 2 was calculated to be 8.8%

Several possible errors could have resulted in inaccurate results in both labs. One possible source of error is that the cuvette, although wiped off before being placed in the colorimeter each time may not have been perfectly clean. Anything left on the cuvette would decrease the light that could pass and would result in a higher measured absorbance. If this happened on the first few solutions of known concentration it would have decreased the slope of the graph, resulting in a larger calculated concentration for the unknown. If this problem had occurred on either of the two more concentrated solutions, it would have increased the slope of the line, causing an decrease in the calculated value for the unknown concentration. If this had occurred with the unknown itself, the higher measured absorbance would have led a calculated concentration that was too high.

Another problem that may have been encountered was that each when each solution was placed in the cuvette, traces of the prior solution may have still been present. For each of the solutions of known concentration, this would have decreased their concentration and therefore their absorbance, lowering the line on the graph. This would have led to a lower intercept value and a higher calculated concentration for the unknown. The unknown was placed in the cuvette after the most concentrated solution was measured and might therefore have been more concentrated, resulting in a higher absorbance and a higher calculated concentration.