[Amanyy]

I have 100ml of water mixed with 25.00ml of 0.02517M k2cr2o7 (m.w=294.2g/mol)to reduce Fe+3 to Fe+2 ..the exess from k2cr2o7 back titration with 5.30ml to 0.0949 Fe+2 ...need to found concentration Fe+2 (55.8g/mol) in ppm ..I don't know how to solve it and how can use 100 water

[chenbeier]

Some errors. In the headline it is written K₂Mn₂O₇. I think you mean K₂Cr₂O₇. Second it is an oxidation of Iron-II to iron-III, not a reduction.


You need the redox reaction. How much mole  iron correspond to the given molarity of the dichromate. With the difference you can find out the original concentration.

[Amanyy]

Ya I know sorry about that mistakes in rewriting..there' 6 mole of Fe+3 and 6mole of fe+2 ..

[Borek]

All back titrations are identical: follow the stoichiometry of the titration to calculate amount of excess substance, subtract the excess from the initial amount and you know how much of the substance reacted.

Once you calculate amount of Fe²⁺ in the initial solution, its volume (100 mL) will be needed to find the concentration.