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    I was provided with a solution of potassium dichromate of unknown concentration and asked to standardise it using a primary standard of ferrous ammonium sulphate. Specifically, I was asked to prepare 500 ml of 0.1M using a solid provided.

    I know the RMM of the solid is 392.13 g/mol, so I work out that to make the solution I need 19.6065 g.

    I didn't weigh that, but rather 19.604 g.

    I then did a rough titration, followed by two more which were concurrent. Took the average of these and it was 15.40 cm^3.

    Actual concentration is 0.09999 mol/dm^3 due to the difference in mass weighed and required.

    I know I need to use the (Md.Vd)/d = (Mf.Vf)/f equation, and as such need to work out the balanced equation.

    I get this as being 1:6 (dichromate:ferrous).

    I titrated against 10 cm^3 of potassium dichromate.

    So the only unknown is the molarity of the potassium dichromate.

    I get the concentration of dichromate as 0.02566 mol/dm^3.

    Is that correct? Because someone took my numbers and ran them through and said they got 0.03850 mol/dm^3, and I can't understand how they got to that.
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    (Original post by ChristopherHuey)
    I was provided with a solution of potassium dichromate of unknown concentration and asked to standardise it using a primary standard of ferrous ammonium sulphate. Specifically, I was asked to prepare 500 ml of 0.1M using a solid provided.

    I know the RMM of the solid is 392.13 g/mol, so I work out that to make the solution I need 19.6065 g.

    I didn't weigh that, but rather 19.604 g.

    I then did a rough titration, followed by two more which were concurrent. Took the average of these and it was 15.40 cm^3.
    concordant*


    Actual concentration is 0.09999 mol/dm^3 due to the difference in mass weighed and required.

    I know I need to use the (Md.Vd)/d = (Mf.Vf)/f equation, and as such need to work out the balanced equation.

    I get this as being 1:6 (dichromate:ferrous).

    I titrated against 10 cm^3 of potassium dichromate.

    Looks OK so far



    So the only unknown is the molarity of the potassium dichromate.

    I get the concentration of dichromate as 0.02566 mol/dm^3.

    Is that correct? Because someone took my numbers and ran them through and said they got 0.03850 mol/dm^3, and I can't understand how they got to that.
    mol iron(II) = 0.0999 x 0.0154 = 1.54 x 10-3

    mol dichromate = 1.54 x 10-4 /6 = 2.57 x 10-4

    molarity dichromate solution = 2.57 x 10-4 / 0.01

    molarity dichromate solution=0.0257 M
 
 
 
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