titrations part 2

Question

The concentration of a Fe²⁺ solution is determined by titrating it with a 0.1259 M solution of dichromate. The balanced net ionic equation for the reaction is shown below.

1.  Cr₂O₇^2-(aq) + 6 Fe²⁺(aq)+14 H₃O⁺(aq)2 Cr³⁺(aq) + 6 Fe³⁺(aq)+21 H₂O(l)

In one experiment, 19.25 mL of the 0.1259 M Cr₂O₇^2- solution is required to react completely with 30.00 mL of the Fe²⁺ solution. Calculate the concentration of the Fe²⁺ solution.

Answer 1

You can use a titration equation for redox if you use incorporate the number of electrons per mole used in the oxidation and reduction. I have never seen this anywhere, but I have taught it for years: e_oM_oV_o = e_rM_rV_r where the subcripts mean the oxidized and the reduced species. The chromate is oxidized and required 6 electrons to go from dichromate to chromium(III) (you can determine this by the change of oxidation state per mole of chromate). The iron(II) changes to iron(III) which requires 1 electron. So the titration equation becomes:

6(.1259M)(19.25ml) = 1(M_Fe(II))(30 ml) (note that this does not require conversion to liters like the stochiometric approach.

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