Name ____________________________________ Finding the % iron in an iron(II) compound
MnO4- (aq)
+
8H+(aq)
+
5Fe2+(aq)  5Fe3+(aq)
+
Mn2+(aq)
+
4H2O(l)
(permanganate ion)
Purpose: To determine the % iron in an unknown iron(II) compound
The permanganate ion (MnO4-) has a dark purple color. All other species in the reaction are colorless.
Pre-Lab
1.
2.
3.
4.
What element is oxidized?
What element is reduced?
What is the mole ratio of Fe2+ to MnO4-?
When mixing permanganate ions with iron (II) ions, how would you know if all the iron (II) ions
have been changed to iron (III) ions?
5. What is the oxidizing agent?
6. What is the reducing agent?
Procedure
1. Obtain about 25 mL of potassium permanganate solution in a small beaker.
2. Use about 5 mL of the potassium permanganate solution to rinse the buret. Collect the rinse in
a waste beaker.
3. Fill the buret with the remaining 20 mL of potassium permanganate solution.
4. Record the initial volume of KMnO4 solution in the buret. (Read the bottom of the meniscus
and report to the correct number of significant digits.)
5. Weigh out about 0.3 grams of the iron(II)ammonium sulfate. Record the exact mass of the
sample. Transfer the sample to an Erlenmyer flask.
6. Add about 25 mL of distilled water to the flask.
7. Measure 5 mL of 6M sulfuric acid and carefully add to the flask. Swirl gently until the solid is
completely dissolved.
8. Position the flask under the buret so the tip of the buret is within the neck of the flask. Place a
piece of white paper under the flask to make it easier to see the color change.
9. Open the buret stopcock and allow 5 - 6 mL of the potassium permanganate solution to flow
into the flask. Swirl the flask and note any color changes.
10. Continue to add the potassium permanganate solution slowly, drop-by-drop, while swirling the
flask. You can rinse any permanganate off the inside walls of the flask using distilled water.
11. When a light pink color persists in the solution while swirling the flask, the end-point has been
reached. Record the final volume of the permanganate solution.
12. Empty the flask into the sink and rinse with tap water followed by distilled water.
13. If there is time do another trial.
14. Clean up
15. Calculate
Name ____________________________________ Finding the % iron in an iron (II) compound
MnO4- (aq)
+
8H+(aq)
+
5Fe2+(aq)  5Fe3+(aq)
+
Mn2+(aq)
+
4H2O(l)
(permanganate ion)
Data table:
Trial 1
Mass of Iron(II)
ammonium sulfate
sample (grams)
Molarity of KMnO4
solution
Initial buret reading
0.025 M
Trial 2
0.025 M
Final buret reading
Volume of KMnO4
added to the flask (mL)
Calculations
Moles of permanganate ion (MnO4-)
added to flask (Show work!)
Moles of Fe2+ in sample
(Show work!)
Mass of Fe2+ in sample
(Show work!)
% iron in the sample
1. What is the formula weight (molar mass) of Ferrous Ammonium Sulfate Hexahydrate
[Fe(NH4)2(SO4)2 · 6H2O]
2. Use the molar mass of iron (Fe) and the molar mass of Fe(NH4)2(SO4)2 · 6H2O to calculate the %
iron in Ferrous Ammonium Sulfate Hexahydrate
3. How does your experimental value for the % iron in the sample (from the table above) compare
to the calculated value from # 2 above.
4. What might explain the difference in the two values?