Chemistry 11 (HL)
Unit 3 / IB Topic 1.5
Determining the Concentration of an
Unknown Copper (II) Sulfate Solution
BACKGROUND INFORMATION
Chemists often need to find the concentration of solutes in a variety of “unknown” solutions. For example,
the concentration of various pollutants in water samples from wells or bodies of water is commonly tested in
many environmental laboratories. The concentration of toxic or banned chemicals in blood or body tissues
is measured in medical and forensic labs.
There are many methods available to measure concentrations of solutions. If the solute has a
characteristic colour when dissolved, then the absorbance of light of a particular colour (or wavelength)
may be measured using a spectrophotometer or a colorimeter.
Recall that white light is made of the three primary light colours – red, blue and green. An object will
appear a certain colour when it reflects or transmits that colour and absorbs the others:
red is absorbed
R
B
blue
object
blue is transmitted and detected by your eye
green is absorbed
G
The darker the colour of an object, the greater the absorbance values of the absorbed wavelengths.
The absorbance reading on its own does not give the concentration of the solution. Instead, it must be
compared to a known reading. One common method to do this involves making a series of solutions of
known concentrations and measuring the absorbance of each one. This data is then plotted to make a
STANDARD CURVE which can then be used to find the concentrations of unknown solutions:
Standard Curve for Bromothymol Blue
absorbance of red light
(absorbance units)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
concentration (mol/L)
If you measure the absorbance of an unknown solution as 0.70 absorbance units, then you could use this
curve to find that it has a concentration of 0.07 mol/L.
p. 1
Chemistry 11 (HL)
Unit 3 / IB Topic 1.5
QUESTION: What are the molar concentrations of two unknown solutions of copper (II) sulfate?
To answer this question, you will:
•
prepare a standard solution of copper (II) sulfate using volumetric flasks
•
use a pipette to accurately measure volumes
•
make a serial dilution series of copper (II) sulfate
•
measure the absorbance of copper (II) sulfate solutions of different concentrations, and make a
standard curve
•
use the standard curve to determine the concentration of two unknown solutions of copper sulfate
PART A: PREPARATION OF A STANDARD SOLUTION OF COPPER (II) SULFATE
Materials:
centigram balance
100 mL beaker
100 mL volumetric flask
glass stirring rod
100 mL reagent bottle
Procedure:
1.
Measure a mass of copper (II) sulfate “by difference”:
a) Measure and record the mass of the vial containing copper (II) sulfate
pentahydrate.
b) Transfer the contents of the vial to the 100 mL beaker.
c) Measure and record the mass of the empty vial.
2.
Add approximately 50 mL of distilled water to the beaker. Stir with the glass rod to
dissolve the solid.
3.
a)
b)
c)
d)
e)
4.
solid copper (II) sulfate pentahydrate (powder)
distilled water (in a wash bottle)
Transfer the solution in the beaker to a volumetric flask.
Rinse the beaker with a small amount of distilled water and transfer to the
volumetric flask. Stir.
Add distilled water until the bulb of the flask is 2/3 -3/4 full. Swirl to mix the
contents.
Add distilled water to the volumetric flask up to the mark on the neck of the flask.
Remember that the bottom of the meniscus should be at the line.
Put the cap on the flask. Make sure it is secure. Hold the cap on the flask and
invert the flask 5-6 times to ensure mixing of the solution.
Transfer the solution to a reagent bottle and label with your name, the solution name
and the date. Save this solution for the next experiment.
Data: Record all relevant data here. Include uncertainties.
p. 2
Chemistry 11 (HL)
Unit 3 / IB Topic 1.5
PART B: PREPARING A DILUTION SERIES OF COPPER (II) SULFATE
Materials:
5 large test tubes
test tube rack
10.0 mL fixed volume pipette
pipette filler
any size beaker
rubber stoppers
distilled water
standard copper (II) sulfate solution (from Part A)
Procedure:
1.
Practice using the pipette as demonstrated by the teacher.
2.
Set up 5 large test tubes in a test rack, labelled 1 to 5.
3.
Use the 10-mL pipette to transfer 10.0 mL of distilled water to each test tube.
4.
Use the 10-mL pipette to transfer 10.0 mL of the standard CuSO4 solution from Part A
to test tube 1. Place a stopper on the test tube and invert several times to mix.
5.
Use the 10-mL pipette to transfer 10.0 mL of the diluted CuSO4 solution in test tube 1
to test tube 2. Place a stopper on the test tube and invert several times to mix.
6.
Use the 10-mL pipette to transfer 10.0 mL of the diluted CuSO4 solution in test tube 2
to test tube 3. Place a stopper on the test tube and invert several times to mix.
7.
Use the 10-mL pipette to transfer 10.0 mL of the diluted CuSO4 solution in test tube 3
to test tube 4. Place a stopper on the test tube and invert several times to mix.
8.
Use the 10-mL pipette to transfer 10.0 mL of the diluted CuSO4 solution in test tube 4
to test tube 5. Place a stopper on the test tube and invert several times to mix.
PART C: MEASURING THE ABSORBANCE OF THE COPPER (II) SULFATE SOLUTIONS
Materials:
Vernier colorimeter
USB link
macbook with Logger Pro
1 cuvette
1 plastic dropper
any size beaker (from Part B)
Procedure:
1.
Connect the colorimeter to the USB link. Connect the USB link to the macbook.
2.
Adjust the colorimeter and Logger Pro settings:
3.
standard copper (II) sulfate solution (Part A)
5 copper (II) sulfate dilutions (Part B)
2 unknown solutions of copper (II) sulfate
a)
Press the < or > arrows on the colorimeter to select a wavelength of 635 nm (red).
b)
In Logger Pro, double click on the “transmittance” window at the lower left. Click
on absorbance in the digital meter options pop up window.
Calibrate the colorimeter:
Do not “START” recording data during the calibration process!
a)
Add distilled water to a cuvette so that it is ¾ full. Cap it. Wipe off the cuvette
with a tissue to remove oils from your fingers. Handle the cuvette only by the cap.
b)
Place the cuvette in the colorimeter and close the lid.
c)
Press the “CAL” button on the colorimeter. The red light underneath the CAL
button should light up to indicate that calibration is occurring.
p. 3
Chemistry 11 (HL)
4.
Unit 3 / IB Topic 1.5
d)
When the red light goes off, click “start” in Logger Pro (green arrow at the top
right). You should see an absorbance reading of 0.000 in the absorbance
window. (If you see a small negative value of approximately -0.002, that is OK.)
e)
You only need to calibrate the colorimeter ONCE in a lab session. Do not touch
that CAL button before each measurement.
Measure the absorbance of all the solutions – the standard solution, the five dilutions
and TWO of the unknown solutions.
a)
You do NOT need to click “Stop” and “Start” between samples. Just leave
the software on record mode.
b)
Use a plastic dropper to transfer the solution to the cuvette so that it is ¾ full.
c)
Wipe off the cuvette before placing it in the colorimeter.
d)
Sometimes the reading will fluctuate or slowly drift. If this is the case, devise a
system for making a reading in the same way for all samples. For example, count
to 5 and then record the value.
e)
Pour out the solution, and shake dry.
f)
Make THREE readings for each solution.
Data: Record your data here.
ANALYSIS
1.
Calculate the concentration of your standard solution in Part A.
2.
Calculate the concentration of each of the dilutions in Part B.
3.
Plot a standard curve using the average absorbance values and the concentrations of the
solutions from Part A and Part B.
NOTE: A standard curve should give a LINEAR plot (even though we call it a curve).
4.
Determine the concentration of the unknown solutions. Show how you found the values.
CONCLUSION and EVALUATION
1.
Make an appropriate conclusion to answer the question on p. 2.
Find the % error.
Comment on the contributions of systematic and random error.
2.
Identify at least two significant weaknesses or limitations in this experiment, and discuss each.
3.
Suggest an improvement for each identified weakness.
p. 4
Chemistry 11 (HL)
Unit 3 / IB Topic 1.5
TEACHER NOTES:
Part A:
Standard CuSO4•5H2O solution
approximately 5.0 g CuSO4•5H2O in 100 mL ~ 0.2 mol/L
Part B:
show how to use the different pipette fillers/bulbs
Part C:
unknown solutions:
A
0.350 M = 21.850 g in 250.0 mL
B
0.042 M = 2.622 g in 250 mL
C
0.018 M = 1.124 g in 250 mL
D
0.145 M = 9.052 g in 250 mL
p. 5