You want to determine the concentration of Fe2+ in a solution. You decide to det

Question

You want to determine the concentration of Fe2+ in a solution. You decide to determine the Fe2+ concentration by means of a redox titration with potassium permanganate. You use 50 mL of the Fe2+ solution and adjust the pH to 1. You need 26.78 mL of a 0.0179 M KMnO4 solution to reach the endpoint. (8 points)

a. What is the reaction that takes place? Write the balanced chemical equation.
b. What is the concentration of Fe2+ in your original solution?
c. What is the balanced chemical equation for the titration of Fe2+ with potassium dichromate? What volume of potassium dichromate (c=0.0148 M) would you have needed for this titration (the pH is acidic)? d. Provide at least two methods to determine the endpoint of the titration with permanganate. Explain exactly how you would determine the endpoint. Do not just state the method.

Explanation / Answer

a.

5Fe2+ + MnO4- + 8H+   = 5Fe3+ + Mn2+ + 4H2O                  n=5 electrons

b.

26.78 mL x 0.0179 M KMnO4 = 5 x 50 mL x CFe2+

CFe2+ = 0.001917 M

c.

Cr2O72-+ 6Fe2+ + 14H+ ® 6Fe3+ + 2Cr3+ + 7H2O           n= 6 electrons

6 x 0.001917 Mx50 mL = V x 0.0148

V = 38.86 mL

d.

1. No indicator is needed: the solution turns from dark purple to a faint pink color at the equivalence point.

2. A potentiometric titration using a Pt electrode and a reference electrode is also possible. A sharp increase of the measured E can be recorded close to EP.

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