Determining Molarity Through Titration of a Redox Reaction

Determining Molarity Through Titration of a Redox Reaction Omar Burkhart Period 2 09/30/2011

Abstract: The goal of this experiment was to find the concentration of Fe2+ solution by using volume ratios taken from titration with MnO41- as well as to determine the difference  between acid base titration and redox titration. In this lab a solution of Mohr’s salt (Fe(NH4)2 (SO4)2*6H2O) is titrated with potassium permanganate (KMnO 4) and used to calculate the concentration of the analyte. A significant finding in this lab was the color  change at the endpoint in which the solution changed a light pink color. When a redox titration reaches its endpoint the color changes due a loss or gain of electrons; in this case MnO41- gains electrons and changes to light pink Mn2+. The natural color change the redox reaction does not require an indicator to determine the end point.

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Purpose: To standardize a solution of Mohr’s salt (ferrous ammonium sulfate, Fe(NH4)2SO4*6H20). Use titration to determine the volume ratio of the Fe2+ solution to the MnO41- standard and calculate the concentration of the Fe2+ solution. Materials:  potassium permanganate (KMnO41-)solution (.020M) •

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ferrous ammonium sulfate hexahydrate (Mohr’s salt, Fe(NH4)2SO4*6H20 solution (concentration unknown, this is the source of the Fe2+ ions)

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6.0M H2SO4

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two 100mL beakers

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10mL volumetric pipet and pipet bulb

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Erlenmeyer flask 

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two burets

Procedure: Put 40 ml of KMnO4 and Fe2+ in separate beakers. Rinse two burets with distilled water then rinse with 5 ml of the solutions. Fill the burrets with the solutions. Remove all air bubbles by emptying some of the solutions into a waste beaker. Add 10 mL of the iron(II) ions to the flask and titrate the solution with the KMnO4 until the it turns a very faint pink color. Record results and repeat as necessary. Data:

Trial #

Volume of Fe2+ (mL) 1 2 3 10.08 10.11 9.99

Volume of KMnO4 (mL) 1 2 3 10.67 10.53 10.18

Calculations: 1) Write a complete, balanced ionic equation for this reaction (MnO4- reduces to Mn2+)

2) Calculate the average volume of each solution used

3) Calculate the moles of MnO4 used based on the average volume.

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4) Calculate the moles of Fe2+ that are in 10.00 mL of solution by using the balanced

equation and the answer to number 3.

5) Calculate the molarity of the Fe2+ solution

6) 39.21g of Fe(NH4)2 (SO4)2 * 6H2O were dissolved in 1.000 L of solution (H2O).

Calculate the actual concentration of the Fe2+ stock solution.

7) Calculate the percent error for your average concentration of the Fe2+ stock 

solution.

Error Analysis: Even with our percent error of .01% there were many factors the affected our data. One source of error was an air bubble in the tip of a buret which would lead to a reading that accounts for more titrant than actually used. Another source of error was the small amount of KMnO4 left on the tip of the buret that didn’t fall into the flask. This would lead to volumes slightly higher than the actual amount. To avoid these errors it would be best to ensure nothing remains on the tip of the buret and that there are no air   bubbles. Conclusions: In this Lab it is apparent that a redox reaction titration is different from a acid  base titration. To start a indicator is not required for a redox reaction because of its natural color change whereas an acid base titration needs an indicator like phenolphthalein. We accomplished the purpose of this experiment and determined the concentration of the iron

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solution. Discussion Questions: 1) Describe the color of each of the reactants and products observed in this experiment a) MnO4- Dark Violet b) H2SO4 Colorless c) Fe2+(in the Fe(NH4)2(SO4)2 * 6H2O solution) yellow with a green tinge d) Mn2+ light pink  2) What was the indicator and the end point color in this experiment and how do you know that this end point is near the equivalence point? The indicator and the end point color in this experiment was light pink which in reality was closer to colorless with just a tinge of pink. This is near the equivalence point because the equivalence point is colorless and the end point has a slight pink hue to it. Since the iron solution is clear the reduction of the manganate changes the color of the solution to pink  when it fully reacts so a light pink hue is the end point. 3) Name at least two other possible oxidizing agents that could be used in this experiment Bismuth(III) and Arsenic(III) 4) In this experiment, H2SO4 was added to the titration flask. Why?

You have to acidify the solution for oxidation and reduction to occur. 5) During the redox reaction in this experiment, the MnO4- was changed to Mn2+. Was

this change oxidation or reduction? Write the half reaction including the correct number of electrons that are involved. Reduction:

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