[scrum]

Okay this one is really weirding me out as I am getting my calcuations from a solutions guide and plugging in #'s and it's still wrong 

A 40.76 mL sample of solution containing Fe2+ ions is titrated with a 0.0112 M KMnO4 solution. It required 22.25 mL of KMnO4 solution to oxidize all the Fe2+ ions to Fe3+ by the following reaction.
MnO4-(aq) + Fe2+(aq)  Mn2+(aq) + Fe3+(aq)       (unbalanced)

(a) What was the concentration of Fe2+ ions in the sample solution?
  M
(b) What volume of 0.0150 M K2Cr2O7 solution would it take to do the same titration? The reaction is shown below.
Cr2O72-(aq) + Fe2+(aq)  Cr3+(aq) + Fe3+(aq)       (unbalanced)

  mL

Okay, for part A I do (.02225 L )X(.0112 mol/L ) X (5molFe/1molMnO4) = .0012 mols

.0012 mol Fe2 / .04076L = .0306 M Fe2+

Correct

For part b

.04076 L X (.0306 mol Fe2+/ L) X (1molCr2O7/6molFe2+) X (1/.0150 mol CrO7) = .0139 ml, which id wrong.

I think 1/.0150 mol CrO7 might be wrong because i don't know where it came from. I'm just substituting my numbers into the equations in the solution books.

[Borek]

First of all - do you know where 5 and 6 came from? Balance both equations.