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Outline a plan of an experiment to determine the percentage of iron present as iron(III) in a solution containing $\ce{Fe^3+(aq)}$ and $\ce{Fe^2+(aq)}$ ions. You are provided with zinc, a standard solution of potassium dichromate(VI) and dilute sulfuric acid. How you would use the zinc and how you would calculate the final answer.

So I know that the following equations will be involved

  • $\ce{2Fe^3+ + Zn -> 2Fe^2+ + Zn^2+}$

  • $\ce{6 Fe^2+ + Cr2O7^2- + 14 H+ → 6 Fe^3+ + 2 Cr^3+ + 7 H2O​}$

I am very unsure on the method to use, I know that I can titrate a sample of the solution against $\ce{K2Cr2O7}$ to eliminate $\ce{Fe2+}$ ions, but I am clueless on how to proceed after.

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  • $\begingroup$ You are provided with $\ce{Zn}$, not $\ce{Zn^2+}$, so you have to find a reaction between $\ce{Zn}$ and either $\ce{Fe^3+}$ or $\ce{Fe^2+}$. $\endgroup$ – orthocresol Jun 10 '16 at 12:33
  • $\begingroup$ You are right, I'll change the equation. $\endgroup$ – daljit97 Jun 10 '16 at 14:06
  • $\begingroup$ So you have a mixture of $\ce{Fe^2+}$ and $\ce{Fe^3+}$ ions, and you know that Zn only reacts with one of them, so maybe you can use that to determine the amount of $\ce{Fe^3+}$ ions? And then what can you use the dichromate to do? $\endgroup$ – orthocresol Jun 10 '16 at 14:54
  • $\begingroup$ Well I can titrate the entire solution against the dichromate to find the amount of Fe2+ ions that reacted. $\endgroup$ – daljit97 Jun 10 '16 at 19:10
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You’re already very close. You have realised that:

  1. You can titrate the $\ce{Fe^{+II}}$ content with a potassium dichromate standard solution; and

  2. You can turn $\ce{Fe^{+III}}$ into $\ce{Fe^{+II}}$ by adding metallic zinc.

Note that in the first bullet point of mine, your second reaction, you are creating $\ce{Fe^{+III}}$, hence iron(III) that was present before you started won’t be modified. Similarly for the other reaction and iron(II). That should lead us to following methodology, hidden in a spoiler tag:

1. Titrate the iron(II) content in an aliquot of your solution. That gives you the content of iron(II) and only iron(II). 2. Take another aliquot and reduce all iron(III) to iron(II). That will give you the total iron content. 3. Substract the value from 1. from the value from 2. to get the content of iron(III).

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