In an acidic aqueous solution, Fe^2+ ions are oxidized to Fe^3+ ions by MnO^-_4:
ID: 502414 • Letter: I
Question
In an acidic aqueous solution, Fe^2+ ions are oxidized to Fe^3+ ions by MnO^-_4: 5Fe^2+ (aq) + MnO^-_4 (aq) + 8H^+(aq) rightarrow 5Fe^3+(aq) + Mn^2+(aq) + 4H_2O(l) Suppose that 1.507 g of FeCl_2 is placed in a 250 mL Erlenmeyer flask to which 50 mL of water and 10 mL of 3 M H_2SO_4(aq) are added. This solution will be titrated by adding 0.1227 M KMnO_4 (aq). a. This is a redox reaction but sulfuric acid is not oxidized or reduced in the reaction. Why is sulfuric acid added to the Erlenmeyer flask? b. How many moles of Fe^2+ are present initially in the Erlenmeyer flask? c. How many moles of MnO^-_4- are required to reach the endpoint in this titration? d. How many mL of 0.1277 M KMnO_4 solution are required to reach the endpoint?Explanation / Answer
a) H2SO4 provides acidic medium for the reaction,
As oxidizing effect of KMnO4 is maximum in acidic medium and least in basic medium as in acidic medium the reduction in oxidation state of Mn is maximum while it is least in basic medium.
8H++MnO4(2-) + 5e(-) ----------> Mn(2+) + 4H2O (acidic medium)
MnO4(-) + e(-) ------------> MnO4(2-) (basic medium)
b) No.of moles = (weight of compound FeCl2) / (Molecular weight of compound FeCl2)=1.507 / 126.751
=0.011889
c) no.of moles of MnO4- = 0.1227 M=0.1227 moles/liter = 0.030675 moles/250 mL
As one mole of MnO4- requires 5 moles of FeCl2,
the limiting agent will be FeCl2.
therefore required moles of MnO4- is, 0.011889/5 = 0.00237 moles.
d) MnO4- concentration is ,0.1277 moles/Liter, but we need 0.00237 moles of MnO4-
so, (1000ml of solution/ 0.1277 moles per liter)*0.00237 moles = 18.55 ml
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