Question

What is the molarity of this dichromate solution?
A potassium dichromate solution of an unknown molarity was used to titrate a solution of FeSO4 prepared by dissolving a 0.350g of pure iron wire in H2SO4 and reducing the FE3+ produced to FE2+. 36.24 mL of the dichromate solution was required to reach an end point. What is the molarity if the dichromate solution?


I THOUGHT I knew how to find unknown molarities, but this whole "reducing the Fe3+ to Fe2+" thing is confusing me.

Answer 1

I don't know why you're confused.
The problem is giving you 0.350 g Fe. mols = grams/atomic mass.
Then you dissolve the Fe in H2SO4. During this process most of the Fe wire goes into solution as Fe^2+ but then some of solution comes into contact with O2 from the air and some of the Fe^2+ is oxidized to Fe^3+ (an old old problem when working with Fe). You could not use this to standardize Cr2O7^2- BECAUSE not all of the Fe is in the +2 state; i.e., some has been oxidized to the +3 state (just another way of saying the iron standard is no longer a good iron standard. What to do? Easy. Add something to reduce the oxidized part BACK to the Fe^2+ THEN titrate with the Cr2O7^2-. That's all that is going on.
1. mols Fe = grams/atomic mass.
2. Write and balance the equation for Fe^2+ + Cr2O7^2- ==> Fe^3+ + Cr^3+
3. Convert mols Fe to mols Cr2O7^2- using the coefficients in the balanced equation.
4. Now M Cr2O7^2- = mols Cr2O7^2-/L Cr2O7^2-