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# Titration watch

1. 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.
3. (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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