Chemistry 11 Chapter 5

PREPARATION OF STANDARD SOLUTIONS AND USE OF A SPECTROPHOTOMETER TO MEASURE THE COPPER(II) CONCENTRATION OF AN UNKNOWN SOLUTION

INTRODUCTION

The purpose of this experiment is to prepare a graph of light absorbance versus concentration and use it to estimate the unknown concentration of copper(II) ions in a solution. First, you and your classmates will prepare solutions containing exactly known concentrations of copper ions. Then you will use an instrument called a SPECTROPHOTOMETER to measure the amount of light absorbed when a beam of light passes through each solution. Finally, you will measure the light absorbed by an unknown solution and use this information to determine the concentration of copper ions in the unknown.

A spectrophotometer works by passing white light through a prism–like device called a DIFFRACTION GRATING. The light is broken up into the colours of the rainbow as shown below.

The diffraction grating can be rotated so that a particular colour ("wavelength") of light is able to pass though a narrow slit and all other colours (wavelengths) are excluded. In this experiment, the light selected will have a wavelength of 660 nm ("nanometers"). The light then passes through a special test tube called a “cuvette” which contains a coloured solution. The light which is not absorbed by the sample then hits a photoelectric cell which in turn produces an electric current. The more light which gets through the sample, the greater the amount of electrical current produced.

The amount of light getting through the solution is called the TRANSMITTANCE, and the amount of light which is absorbed by the sample is called the ABSORBANCE. Anything which blocks the light from getting through the test tube, such as coloured ions or stray flecks of unwanted solid particles, will increase the absorbance of the sample. In this experiment you will measure the ABSORBANCE because there is a simple relationship between absorbance and concentration: doubling the concentration will double the absorbance. Therefore, a graph of absorbance versus concentration should give a straight line. (Note: your data points may not lie on a perfectly straight line.)

In the graph below, the data points represent known concentrations. Such a graph is called a CALIBRATION GRAPH because it allows you to establish or "calibrate" the absorbance versus concentration behaviour of the chemical used. As can be seen, the points fall on a straight line.

Purpose

  • To practice a molarity calculation by preparing a standard solution
  • To practice dilution calculations by preparing a set of solutions with specific

concentrations from the standard solution.

  • To use a spectrophotometer

Materials

  • Copper(II) sulphate (fine reagent grade crystals) in a beaker
  • 500mL of 1% nitric acid
  • 250 mL volumetric flask and rubber stopper
  • a dropper bottle filled with 1% nitric acid
  • centigram balance
  • 10 mL and a 25 mL graduated cylinder
  • scoopula
  • eye dropper
  • 8 test tubes labeled 1 – 8

Procedure

1. Prepare the STANDARD SOLUTION.

Prepare 250.0 mL of a 0.200 M solution of CuSO4•5H2O. Be sure to show your calculation to your teacher before weighing the CuSO4•5H2O.

Use a clean dry weighing boat to weigh out the mass of CuSO4•5H2O that you calculated.

IMPORTANT: Do NOT try to get the exact mass. Instead, settle for ANY mass within ±0.1 g of your calculated mass and record the actual mass you obtained to 3 decimal places in Table I.

Carefully transfer ALL of the CuSO4•5H2O into a 250.00 mL beaker by using a wash bottle filled with 1% nitric acid to wash the solid from the weighing boat into a beaker and thoroughly wash off the boat. Then transfer the solution into a 250.0 mL volumetric flask. [CARE: don't get the acid on yourself. It isn’t very corrosive but can make your skin itchy.]

Take about 250 mL of 1% nitric acid solution provided and use the acid to half fill the flask. Stopper and shake the flask until the solid is completely dissolved. Carefully remove the stopper (don't lose any liquid), fill the flask almost full of acid solution and use a dropper to get the bottom of the meniscus up to the line around the neck of the volumetric flask.

2. Prepare the set of solutions

It is not necessary to measure the volume of the standard solution exactly. Be sure to record the actual volume used in the Table 1.

Solution #1

Measure 5.0 mL of the standard solution in 25.0 mL graduated cylinders. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #1 and place the test tube in a test tube rack.

Solution #2

Measure 7.0 mL of the standard solution in 25.0 mL graduated cylinders. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #2 and place the test tube in a test tube rack.

Solution #3

Measure 10.0 mL of the standard solution in 25.0 mL graduated cylinder. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #3 and place the test tube in a test tube rack.

Solution #4

Measure 13.0 mL of the standard solution in 25.0 mL graduated cylinder. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #4 and place the test tube in a test tube rack.

Solution #5

Measure 17.0 mL of the standard solution in 25.0 mL graduated cylinder. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #5 and place the test tube in a test tube rack.

Solution #6

Measure 20.0 mL of the standard solution in 25.0 mL graduated cylinder. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #6 and place the test tube in a test tube rack.

Solution #7

Measure 23.0 mL of the standard solution in 25.0 mL graduated cylinder. Fill the graduated cylinder up to the 25.0 mL mark with 1% nitric acid.

Pour this solution into a clean, dry test tube, labeled #7 and place the test

tube in a test tube rack.

Solution #8, this will be your standard solution.

Pour this solution into a clean, dry test tube, labeled #8 and place the test

tube in a test tube rack.

3. Measure the Absorbance using the Spectrophotometer

Once you have made all of the solutions it is time to pour a small amount of each solution into a small test tube (called a cuvette) that can fit into the spectrophotometer. Be sure to wash out the small test tube with a small amount of your solution two times before finally filling the test tube 2/3 full.

The spectrophotometer has been set up by your teacher and is calibrated.

Place a cuvette filled with nitric acid into the spectrophotometer. It will be on the side of the spectrophotometer.Wipe the side of the cuvette with a “Kim Wipe” to remove any fingerprints from the sides of the cuvette. This procedure allows the cuvette and nitric acid solution to act as a "blank".

Place the 8 solutions in the spectrophotometer and record the absorbance of each in Table 1.

Measure the absorbance of the unknown solution that has been assigned by your teacher. The unknowns will be on the side of the spectrophotometer in a cuvette.

Data and Observations

Actual Mass of CuSO4used to make standard solution - ______g

Concentration of your standard solution - ______M

Table 1. Concentrations and Absorbance of the Solutions and Unknown

Volume of standard solution (mL) / Concentration of CuSO4 (M) = Concentration of Cu+2 / Absorbance
(%)
Solution #1
Solution #2
Solution #3
Solution #4
Solution #5
Solution #6
Solution #7
Solution #8
Unknown ____

ANALYSIS OF DATA

1a.Show how you calculated the mass of CuSO4•5H2O required to make 250.0mL of a 0.200M solution

1b. Calculate the ACTUAL [CuSO4•5H2O] in each of the known solutions. The [Cu2+] in each solution is equal to the [CuSO4•5H2O] in the solution. Place the results in Table I.

2.Plot a graph of Absorbance, on the vertical axis versus Concentration(M) of Cu+2 on the horizontal axis. Draw the best line through the data points.

Be sure to include: proper title

properly selected and drawn axes

proper plotting

proper “best line”, the line should be straight

3.Draw a horizontal line on the graph at the absorbance of your unknown so that the line intersects the line through the data points. Next draw a vertical line down to the concentration axis from the point where the horizontal line intersects the data line. Label the coordinates of the intersection point on the graph. In the body of your written report, state the concentration of CuSO4•5H2O found for your unknown and the unknown letter used.

QUESTIONS

1.If you had accidentally added about 10 mL too much 1% nitric acid to the original solution you made in the volumetric flask, would your absorbance reading have been too high or too low as a result? Explain your answer completely.

2.If the CuSO4•5H2O you used had been contaminated with another chemical which caused a large number of solid, opaque particles to float around in the solution, would your absorbance reading have been to high or too low as a result? Explain your answer completely. [Hint: re–read the introduction.]

3.Assume you are hired to work in the lab of a chemical manufacturing plant and are asked to set up a procedure that will allow you to check the concentration of aqueous Co2+ ions (pink-red) in a reaction vessel every 15 minutes. How could you use the ideas and procedures in this experiment to set up a testing procedure, from scratch, that allows you to quickly and accurately determine the concentration of cobalt ions in the solutions that arrive at your lab every 15 minutes?

4.The lab aide made up 750.0 mL of each unknown solution. What mass of CuSO4•5H2O did she need to make up the unknown solution you selected?

DISCUSSION

Please state the following in your discussion

  • summarize the procedure
  • summarize your results, are they correct?
  • Did your data yield good results? Is your graph a straight line?
  • List and explain any errors in your results.
  • What is the calculated concentration of the unknown solution
  • Is your calculated unknown concentration similar to the actual concentration of the unknown. Why or why not?
  • Would you do anything differently next time?

CONCLUSION

Please state the following in your conclusion

  • State that you completed the objectives, if you actually completed all of them
  • State the letter and the concentration of your unknown solution and the actual concentration of the unknown
    Answer Key

Unknown / Volumes of 0.200M
CuSO4 5H2O / Concentration of CuSO4 5H2O / Absorbance
A
B
C
D

QUESTIONS

1.If you had accidentally added about 10 mL too much 1% nitric acid to the original solution you made in the volumetric flask, would your absorbance reading have been too high or too low as a result? Explain your answer.

(1/2)Too low

(1)The additional volume would have diluted the original solution and decreased its concentration. Since the greater the concentration, the larger the absorbance found, the low concentration would lead to a low absorbance.

2.If the CuSO4•5H2O you used had been contaminated with another chemical which caused a large number of solid, opaque particles to float around in the solution, would your absorbance reading have been to high or too low as a result? Explain your answer.

(1/2)Too high

(1)The floating particles would block more light and increase the light absorbed (less light gets to the photocell)

3.Assume you are working in a chemical manufacturing plant and have to check the concentration of aqueous Co2+ ions (pink-red) in a reaction vessel every 15 minutes. How could you use the ideas and procedures in this experiment to quickly and accurately determine the concentration of the cobalt ions in the solution?

(1)•Make a calibration chart of absorbance versus concentration for various concentrations of cobalt ions.

(1)•Measure the absorbance of the samples from the reaction vessel every 30 minutes, and read off the concentration from the calibration curve.

Actual [Cu2+] / Absorbance / Actual [Cu2+] / Absorbance
0.020 / 0.060 / 0.120 / 0.333
0.0401 / 0.118 / 0.140 / 0.382
0.0601 / 0.176 / 0.160 / 0.426
0.0801 / 0.230 / 0.180 / 0.472
0.100 / 0.284 / 0.200 / 0.516