1
Redox Stoichiometry (Titrations)
·
·
In a redox titration, a known concentration of an oxidizing agent is used
to find an unknown concentration of a reducing agent, or vice-versa.
Applications include finding the iron content of water or the Vitamin C
content in foods.
General Steps:
1.
Balance the redox equation
2.
Organize data for the given and required substances
3.
Find the moles of the given substance
If given volume and concentration, use n=Cv
If given mass, find molar mass, then use n= mM
4.
Find the moles of the required substances, using mole ratio
5.
Convert moles to concentration (C=n/v) or volume (v=n/C)
or mass (m=nM) of required substance
Example 1:
The following example involves the titration of a solution of sodium oxalate,
Na2C2O4(aq) with a solution of potassium permanganate, KMnO4(aq).
Important Note:
·
The permanganate ion, MnO4-, is often used as an oxidizing agent
because of its deep purple color.
·
It acts as its own indicator.
·
It is reduced to pale pink Mn2+ by the oxalate ions.
·
Once all oxalate ions have been oxidized the next drop of permanganate
ions turns the solution a faint purple color, signaling the endpoint of the
titration.
Find the volume of 0.200 M solution of MnO4- which will react with 50.0 mL of
0.100 M solution of C2O42-.
MnO4- + C2O42-  Mn2+ + CO2
Example 2:
A 23.30 ml sample of KMnO4 solution is decolorized by 0.1111g of oxalic acid
(H2C2O4). The products are Mn2+ and CO2 gas. Calculate the concentration of
KMnO4 using the following equation:
Example 3:
The oxidation of Fe2+ ions by permanganate in acidic solution is accompanied
by the formation of Fe3+ ions and Mn2+ ions. How many moles of FeSO4 would
be oxidized by 100.0 ml of 0.02118 mol/L KMnO4 solution.
Fe2+ + MnO4-  Fe3+ + Mn2+
2
ELECTROCHEMISTRY WORKSHEET #3
1.
In a redox titration 12.50 mL of 0.0800 mol/L K2Cr2O7 (aq) was used in
acidic solution to oxidize Sn2+ (aq) ions to Sn4+ (aq) ions. The volume of
K2Cr2O7 (aq) used was just sufficient to oxidize all the Sn2+ (aq) in 10.0 mL
of the solution. Calculate the concentration of the Sn2+(aq) ions in the
solution according to the following unbalanced equation.
(Ans: 0.300 mol/L)
22+
4+
3+
Cr2O7 (aq) + Sn (aq)  Sn (aq) + Cr (aq)
2.
The copper (II) ions in a solution can be converted to copper metal by
trickling the solution over scrap iron. The reaction produced iron (II) ions
from scrap iron. If the process produces 25.00 L of solution containing
0.00200 mol/L of Fe2+(aq) ions, what mass of copper is produced?
(Ans: 3.18g)
Cu2+(aq) + Fe(s)  Fe2+(aq) + Cu(s)
3
3.
What volume of 0.0500 mol/L KmnO4 (aq) is needed to oxidize all the Br(aq) ions in 25.0 mL of an acidic 0.200 mol/L NaBr(aq) solution according to
the following unbalanced equation.
(Ans: v = 20.0mL)
2+
MnO4 (aq) + Br (aq)  Br2 (aq) + Mn (aq)
4.
Aqueous solutions of hydrogen peroxide sold in pharmacies are usually
approximately 3% H2O2 by mass. However, in solution, hydrogen peroxide
decomposes into water and oxygen.
What is the percent by mass of a solution of hydrogen peroxide, H2O2,
prepared from 1.423 g of H2O2 which is titrated with 40.22 mL of 0.01143
mol/L KMnO4(aq).
The reaction occurs in an acidified solution.
(Hint: Find mass of H2O2 actually present, then mass %)
2.747%)
(Ans:
Balanced equation:
5H2O2(aq) + 2MnO4-(aq) + 6H+(aq)  2Mn2+(aq) + 5O2(g) + 8H2O(l)
Page 744: #’s 29 - 32