Concentration of an Aqueous Soultion

The concentration of an aqueous solution of NaOCl (sodium hypochlorite; the active ingredient in household bleach) can be determined by a redox titration with iodide ion in acidic solution: OCl−(aq)+2I−(aq)+2H+(aq)→Cl−(aq)+I2(aq)+H2O(l)Assume that the blue spheres in the buret represent I− ions, the red spheres in the flask represent OCl− ions, the concentration of the I−ions in the buret is 0.140M , and the volumes in the buret and the flask are identical.

What is the concentration of NaOCl in the flask?

What percentage of the I− solution in the buret must be added to the flask to react with all the OCl− ions?

 

Balancing Redox Equations in Acidic Solution

To balance oxidation-reduction equations that occur in solution.

It is one thing to balance an oxidation-reduction (redox) reaction when you are given the full unbalanced equation, and quite another when you are given only certain parts of it. In this problem, you will be given some information about an oxidation-reduction reaction and you will have to come up with the full balanced equation.

For example, say you know that SO2 is oxidized to SO42− and Br2 is reduced to Br−in acidic solution. From this information you can determine the half-reactions to be as follows:

SO2→SO42−+2e−

2e−+Br2→2Br−

And since there is a gain or loss of two electrons in each of these half-reactions, you might be inclined to write the full equation like this:

SO2+Br2→SO42−+2Br−

But even though the electrons are balanced, oxygen is not balanced. What has been missed? 

The secret to this problem lies in the fact that the reaction occurs in acidic solution. This means that H2O is present as well as an excess of H+ ions. Therefore you should add H+ and H2O to the equation where appropriate. For example,

SO2+Br2→SO42−+2Br−

needs two more O atoms on the left. Therefore you must add 2H2O to the left side of the equation to get

2H2O+SO2+Br2→SO42−+2Br−

Now you need four more H atoms on the right, so you should add 4H+ to the right side of the equation to get

2H2O+SO2+Br2→SO42−+2Br−+4H+

Now the equation is properly balanced.

Note that, in basic solution, H2O and OH−are used to balance the equation instead of H2O and H+.

In acidic solution, MnO4− is reduced to Mn2+ , whereas Fe2+ is oxidized to Fe3+

Given the partial equation

? MnO4− + ? Fe2+ → ? Mn2+ + ? Fe3+

what must the coefficients be so that the electrons are balanced?