AP Chem Lab Chapter 4: Determination of Iron by Reaction with Permanganate -A Redox Titration
KMnO4 will be employed in this experiment to determine the percentage of iron in an unknown containing iron(II) ammonium sulfate, Fe(NH4)2(SO4)2 á 6 H2O. The mass of iron in the sample must be first determined from the redox reaction stoichiometry.
Potassium permanganate, KMnO4, is widely used as an oxidizing agent in volumetric analysis. In acid solution, MnO4- ion undergoes reduction to Mn2+ as shown in the following equation:
8 H+(aq) + MnO4- (aq) ˆ Mn2+ (aq) + 4 H2O
The titration, which involves the oxidation of Fe2+ ion to Fe3+ by permanganate ion, is carried out in sulfuric acid solution to prevent the air-oxidation of Fe2+. The end point of the titration is sharpened markedly if phosphoric acid is present, since the Fe3+ ion produced in the titration also forms an essentially colorless complex with the acid. Since both the Fe3+ and Mn2+ ions are nearly colorless, the end point can be taken as the first permanent pink color that appears in the solution after the addition of the phosphoric acid. The overall reactions are as follows:
Fe compound + H2SO4 ˆ Fe2+ + MnO4- ˆ Fe3+ + Mn2+ ---- > pink endpt
(prevents air oxidation) (analyte) (titrant) (lt.yellow) H3PO4
For the calculations: the number of moles of potassium permanganate used in the titration is equal to the product of the molarity of the KMnO4 and its volume used. The number of moles or iron present in the sample is obtained from the balanced equation for the reaction and the amount of MnO4- ion reacted. The percent by weight of iron in the solid sample follows directly.
molarity MnO4- ˆ moles MnO4- ˆ moles Fe2+ ˆ grams Fe ˆ % Fe in sample
Buret Unknown iron(II) sample Analytical balance
Buret clamp 250 mL Erlenmeyer flask 0.02M KMnO4
Ring stand 85% H3PO4 1M H2SO4
Magnetic stirrer & stir bar
1. Obtain a buret, buret clamp, ring stand, magnetic stirrer & stir bar.
2. Weigh out accurately on the analytical balance a sample of about 1.0 g of the unknown iron(II) sample and put it into a clean 250-ml Erlenmeyer flask. Record four decimal places since the analytical balance measures to the 10,000th of a gram.
3. Clean your buret thoroughly: In a beaker, obtain about 50 ml of the standard KMnO4 solution from the carboy or bottle in the laboratory. Rinse the buret three times, each with about 5 milliliters of the KMnO4. Drain and then fill the buret with rest of the KMnO4 solution up to the top line (0 mL).
4. Attach the buret clamp to the ring stand and place the magnetic stirrer on the stand. Attach the filled buret onto the buret clamp such that it sits above the magnetic stirrer.
5. Obtain 50 ml of the 1 M H2SO4 and add it to the iron sample. The sample should dissolve completely. Then without delay, titrate this iron solution with the KMnO4 solution. When a light yellow color develops in the iron solution during the titration, add 3 ml of 85% H3PO4. CAUTION: Caustic reagent. Continue the titration until you obtain the first pink color that persists for 15 to 30 seconds.
Mass of sample weighed = g
(original iron II sample)
Initial buret reading = mL KMnO4
Final buret reading = mL KMnO4
(pink end point)
Calculations (show work*):
*Volume KMnO4 required = mL KMnO4
(difference between final & initial)
*Moles KMnO4 required = moles KMnO4
(Molarity x volume)
*Moles of Fe2+ in sample = moles of Fe2+
(mole ratio from KMnO4 to Fe)
*Mass of Fe in sample = g
(moles x molar mass)
* %Fe in sample = %
(mass Fe/mass sample x 100)
**Measure to the nearest: Mass- thousandth of a gram Volume- tenth of a ml
1. Advance Study Assignment: Determination of Iron by Permanganate Titration
Fe(NH4)2(SO4)2 * 6 H2O? (Hint: Use molar masses of each element)
2. Lab FRQ – Redox Titration
Potassium permanganate, KMnO4, is purple colored. When it reacts completely with hydrogen peroxide, H2O2, in an excess of sulfuric acid, H2SO4, colorless products are formed. The reaction is as follows:
5 H2O2 + 2 KMnO4 + 3 H2SO4 ˆ K2SO4 + 2MnSO4 + 8 H2O + 5O2
unknown concentration of H2O2, hydrogen peroxide distilled water
0.1M KMnO4 (purple colored) Erlenmeyer flask
1.0M H2SO4 (in excess) buret, ring stand, & buret clamp
1. List the data that needs to be collected in order to determine the concentration (molarity) of the hydrogen peroxide.
2. List the calculations that need to be performed in order to determine the molarity of the hydrogen peroxide.
3. The buret should be rinsed out with the titrant solution. How would the results change if this was not done?
3. Lab FRQ – Redox Stoichiometry
5Fe2+ (aq) + MnO4– (aq) + 8H+ (aq) ˆ 5Fe3+(aq) + Mn2+(aq) + 4H2O (l)
The mass percent of iron in a soluble iron (II) compound is measured using a titration based on the balanced equation above.
1. What is the oxidation number of manganese in permanganate ion, MnO4 – (aq)?
2. Identify the reducing agent in the reaction represented above.
The mass of a sample of the iron (II) compound is carefully measured before the sample is dissolved in distilled water. The resulting solution is acidified with H2SO4 (aq). The solution is then titrated with MnO4 - (aq) until the end point is reached.
3. Describe the color change that occurs in the flask when the endpoint of the titration has been reached. Explain why the color of the solution changes at the end point.
4. Let the variables g, M, and V be defined as follows:
g = the mass , in grams, of the sample of the iron(II) compound
M = the molarity of the MnO4 - (aq) used as the titrant
V = the volume, in liters of MnO4 - (aq) added to reach the end point
In terms of these variables, the number of moles of MnO4 - (aq) added to reach the endpoint of the titration is expressed as M x V. Using the variables defined above, the molar mass of iron (55.85 g mol –1), and the coefficients in the balanced chemical equation, write the expression for each of the following quantities.
(i) The number of moles of iron in the sample
(ii) The mass of iron in the sample, in grams
(iii) The mass percent of iron in the compound
5. What effect will adding too much titrant have on the experimentally determined value of the mass percent of iron in the compound? Justify your answer.
Chapter 4 AP Lab FRQ – Redox Titration & Stoichiometry
Oxalic acid, H2C2O4, is a primary standard used to determine the concentration of potassium permanganate, KMnO4, in solution. The equation for the reaction is:
2KMnO4(aq) + 5H2C2O4(aq) + 3 H2SO4(aq) ˆ 2 MnSO4(aq) + 10CO2(g) + 8H2O(l) + K2SO4(aq)
A student dissolves a sample of oxalic acid in a flask with 30mL of water and 2.00mL of 3.00M H2SO4. The KMnO4 solution unknown concentration is in a 25.0 mL buret. In the titration, the KMnO4 solution is added to the solution containing oxalic acid.
a. What chemical species is being oxidized in the reaction?
b. What substance indicates the observable endpoint of the titration? Describe the observation that shows the endpoint has been reached.
c. What data must be collected in the titration in order to determine the molar concentration of the unknown KMnO4 solution.
d. Without doing any calculations, explain how to determine the molarity of the unknown KMnO4 solution.
e. How would the calculated concentration of the KMnO4 solution be affected if 40mL of water was added to the oxalic acid initially instead of 30mL? Explain your reasoning.