[Lozia]

I have done an a back titration experiment using Mohr's salt and dichromate. Firstly I standardised some KMnO₄ to which I got the concentration of 0.0195mol L^-1. I then used the equation to find the concentration of my Mohr's salt.

MnO₄ + 5Fe²⁺ + 8H⁺ --> Mn²⁺ + 5Fe³⁺ + 4H₂O

I got the concentration to be 0.0894mol L^-1

The equation for the dichromate is:

Cr₂O₇^2- + 6Fe²⁺ +  14H⁺ --> 2Cr³⁺ + 6Fe³⁺ +  7H₂O

For the titration where I had both Mohr's salts and dichromate together I got: 20.9cm3 and 21.0cm3.

If any more info is needed let me know

Its not just a case of saying 6 Mohr's salts react with 1 dichromate, so 0.002 moles of mohr's salts reacts with 0.000333moles of dichromate? Then working concentration out from the volume and moles?

[Borek]

Honestly - I have no idea what you did and what you are looking for. Describe your procedure in more details.

[Lozia]

Lol, Ok, reading that back, I can see that I haven't explained to well.

The procedure or the aim of the experiment was to determine the concentration of the dichromate using back titration. The first thing I did was to standardise potassium permanganate to find the exact concentration of it. I then titrated a mixture of dichromate and mohr's salts with the standardised potassium permanganate. I then did a blank titration which was just using the mohr's salt and not dichromate. So I know the concentration of the mohr's salt and the potassium permanganate. I just somehow have to use these to find the concentration of the dichromate.

[Borek]

OK, so you have to find out how much Mohr salt was 'eaten' by the added dichromate. That should be easy - you are right about 1:5 ratio of Fe²⁺ and permanganate, you are right about 1:6 ratio for Fe²⁺ and dichromate - that's all simple stoichiometry

[Lozia]

Sooo, 0.0004 permanganate reacts with 0.002 mohr's salt. Then 0.002moles mohrs salt with 0.000333 dichromate. So then, to find concentration, I just do 0.000333/(volume i titrated)?

Thank you.