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.
Chemistry 11
Chapter 5
Purpose
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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
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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.
Chemistry 11
Chapter 5
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.
Chemistry 11
Chapter 5
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 CuSO4 used 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)
Solution #1
Solution #2
Solution #3
Solution #4
Solution #5
Solution #6
Solution #7
Solution #8
Unknown ____
Concentration of
CuSO4 (M) =
Concentration of Cu+2
Absorbance
(%)
Chemistry 11
Chapter 5
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?
Chemistry 11
Chapter 5
DISCUSSION
Please state the following in your discussion
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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
Chemistry 11
Chapter 5
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)
(1)
Too low
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)
(1)
Too high
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)
(1)
• Make a calibration chart of absorbance versus concentration for various
concentrations of cobalt ions.
• 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