AP Chemistry Laboratory Investigation
Determination of the Actual Percentage of H2O2 in a Drugstore Bottle of
Hydrogen Peroxide
Objectives:
1. Standardize a solution to be used in a redox titration
2. design and interpret the results of an experiment involving a redox titration
3. determine the concentration of an analyte in a solution
Central Challenge:
This lab has two major tasks. The first task is to standardize the concentration of a KMnO4 solution.
This task is necessary in order to complete the second task, which is to evaluate how close
commercial H2O2 solutions are to their labeled concentrations. Different groups of students will
work with different brands and then share their results.
Context for this Investigation:
Container labels provide detailed information about the contents present in a given container. Who
determines that information? Are there mechanisms taken by manufacturers to verify the
information present on those labels? These are questions you will address in this lab.
Information to Strengthen Student Understanding:
Oxidation–reduction (redox) reactions involve a transfer of electrons between the species
being oxidized and the species being reduced. The reactions are often balanced by separating the
reaction components into two half-reactions: oxidation (loss of electrons) and reduction (gain of
electrons). In a redox reaction, the number of electrons lost by the species being oxidized is always
equal to the number of electrons gained by the species being reduced.
The Permanganate ion is commonly used as a titrant in redox reactions because as it is reduced (in
4
an acidic solution), it changes from a purple color to colorless. When the titration is just past the
endpoint, the solution will not remain colorless. You need to stop the titration when the solution is
just barely pink. You will first standardize a permanganate ion solution, then you will use the
standardized solution in a titration to determine the concentration of an analyte (component in a
solution).
Standardization: It is difficult to make up a solution of KMnO4 to an accurate concentration,
as solid KMnO4 often contains impurities such as chlorides, sulfates, and nitrates. KMnO4 can be
standardized in acidic solution with a known concentration of iron (II) ions. In this oxidation–
reduction reaction, manganese (II) ions and iron (III) ions are formed. The balanced equation for the
reaction is:
MnO4- + 5 Fe2+ + 8 H+ → Mn2+ + 5 Fe3+ + 4 H2O
Peroxide titration: Hydrogen peroxide, H2O2, is easily oxidized. It is used in commercial
bleaching processes and in wastewater treatment plants as an environmentally friendly alternative
to chlorine. Dilute solutions of H2O2 are used to bleach hair and to clean wounds. It readily
decomposes in the presence of light, heat, or metallic catalysts into water and oxygen. It is
important to know the actual concentration of a solution of H2O2 as its effectiveness can decrease
with smaller concentrations. In the reaction being studied in this lab, solutions of hydrogen
peroxide, H2O2, and potassium permanganate, KMnO4, will be combined in an acidic solution. The
balanced equation for the reaction is:
2MnO4-(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 5O2(g) + 8H2O(l)
Store bought hydrogen peroxide claims to have a concentration of 3%. That value means
that 3g of hydrogen peroxide is in 100ml of solution (or 0.03g/ml). This is equivalent to 0.88M.
Pre-Lab Assignment: Complete the following pre-lab assignment while referring to the
“Information to Strengthen Student Understanding” section. It will help prepare you for developing
your own procedure, and analyzing your data.
1.
2.
3.
4.
5.
6.
7.
8.
What measuring devices in the materials list (on the next page) are used to obtain very
accurate measurements of volume? What measuring devices are used to obtain
approximate measurements of volumes?
Look at the balanced equation for the reduction of permanganate ions by iron (II) ions.
How many electrons are transferred in this reaction?
What is the mole ratio between the permanganate ions and the iron(II) ions in the reaction?
As you add a permanganate solution (the titrant) to an acidified iron(II) solution, what color
change will you observe at the endpoint?
Why are you performing the standardization of permanganate titration?
Look at the balanced reaction for the reduction of permanganate ions by hydrogen
peroxide. How many electrons are transferred in this reaction?
As you add the standardized permanganate solution (the titrant) to your peroxide solution,
what color change will you observe at the endpoint?
Practice calculation: A sample of oxalic acid, H2C2O4, was analyzed using a standardized
solution of KMnO4. 25.0 mL of oxalic acid was used in the titration. 12.30 mL of a 0.0226M
KMnO4 was added to the sample when a faint pink color was observed. The balanced
equation for this reaction is shown below:
6H+(aq) + 2 MnO4 –(aq) + 5 H2C2O4 (aq) → 10 CO2(g) +8 H2O(l) + 2Mn2+(aq)
a.
How many moles of MnO4- ions were reduced in the reaction?
b.
How many moles of H2C2O4 are present in the sample?
c.
What is the molarity of the oxalic acid solution?
d.
What does the sample turning a faint pink color indicate about the reaction?
Materials:
The following are available for you use. You may not need ALL of these materials:
Chemicals
~0.02M KMnO4
0.100M ammonium iron(II) sulfate
(Fe(NH4)2(SO4)2·6H2O)
Various 3% hydrogen peroxide solutions
6M H2SO4
Distilled water
Equipment
assorted sizes of beakers
10ml graduated pipet
assorted sizes of Erlenmeyer flasks
50ml buret
assorted sizes of graduated cylinders
Buret clamp
Ringstand
Vernier magnetic stirrer
A micropipet (adjusts from 100µl to 1ml)
Procedure guidelines:
Although you will be coming up with your own exact procedure for this lab, here are some
guidelines to follow:
1. You will be performing the standardization of permanganate titration first. Your titrant will
be the permanganate solution, and your sample will be the acidified iron(II) solution. You
will only do 1 trial of this, but we’ll use a class average for calculations involving the peroxide
titration.
2. 10ml of the 6M H2SO4 is plenty to acidify the solution
3. The exact volume you use of your iron(II) solution doesn’t matter, as long as you have room
in your flask to add the H2SO4 and the titrant. Just make sure you measure the volume
accurately, using very accurate volume measuring equipment. For the H2O2 titration, you
will want to use only 1-2ml of H2O2, since the H2O2 is approximately 44 times as
concentrated as the permanganate (0.88M vs 0.02M). Again, measure that volume very
accurately (use a micropipet), and add some dH2O to the flask so the volume is greater.
4. If time allows, do multiple trials of the peroxide titration
Data:
Create a chart for the values you should be recording for each titration.
Calculations:
For the permanganate standardization:
1. Calculate the moles of iron(II) ions in the sample
2. Calculate the moles of KMnO4 used in the titration (refer to the equation in the information
section for the mole ratio to use)
3. Calculate the molarity of the KMnO4 solution
4. Record the molarity that represents the average for the whole class
For the peroxide titration:
5. Calculate the moles of KMnO4 used for each trial
6. Calculate the moles of H2O2 in the sample for each trial (refer to the equation in the
information section for the mole ratio to use)
7. Calculate the molarity of the H2O2 sample for each trial
8. What is the average molarity of your H2O2 sample?
Conclusion Questions:
1. Was your calculated molarity of the H2O2 solution you tested different than the molarity
claimed (0.88M)? If so, was it higher or lower?
2. Do you think your results are reliable? Explain your answer.
3. How would your calculated concentration of the H2O2 solution been affected (i.e. would it
be too high or too low) if when you stopped the titration, the solution in the flask was dark
purple instead of very light purple? Explain your answer.