Needing some help here please, in chemistry we have been determining the exact mass of iron in women's health iron tablets (Wilkos own). the bottle states each tablet contains 14mg of iron.
Steps: crush up 5 iron tablets and dissolve in sulfuric acid, I believe the concentration and amount is not important, it is just to stop the iron from oxidising.
make up a standard solution of 250cm^3 .... KMnO4/H+ 0.02M
when running through the following calculations it should titrate with 1.2cm^3 of Pottasium permangante solution (acidified) however the actual mean titre is 12cm^3, 10x more.
5 x 14mg = 70mg (original mass - mass lost/original mass) x iron content = 67x10^-3 g 67x10^-3/56(RMM) = 1.196x10^-3 mol 1.196x10^-3 x 1000/250 = 4.786x10^-3 moldm^3 (iron solution) 4.786x10^-3 x (25/1000) = 1.1965x10^-4 mol (used for one titration) (5 iron ions wirth one manganate ion) 1.1965x10^-4 / 5 = 2.393x10^-5 mols 2.393x10^-5 / 0.02 = 1.1965x10^-3 dm^3 of KMnO4 solution dm^3 ---> cm^3 (x1000) 1.1965x10^-3 x 1000 = 1.1965 cm^3
Obviosuly wilkos arent going to bewrong on the mass of iron in each tablet (iron sulpahte i believe). so me the teacher and a friend in the class have ran through this calculation multiple times and still cannot find a mistake. the next logical thought was that the KMnO4/H+ solution was not actually 0.02M but 0.002M
So we have now titrated the KMnO4 solution with a solution of iron sulpahte we made up our selves. Mr of iron sulphate = 278
We made up a 0.1M solution of iron sulphate by dissolving+ 2.78g of iron sulphate into 100cm^3 solution.
The mean titre was 25cm^3
0.1 x (25/1000) = 2.5x10^-3 mol
same as before 1:5 reaction
2.5x10^-3 / 5 = 5x10^-4 mol in 25cm^3 therefore
1000/25 x 5x10^-4 = 0.02M KMnO4 solution.
I'm pretty much certain all the maths is correct (although not all the grammar and spelling is in this post) as 3 of us have checked it over multiple times. any suggestions to what could be going wrong? Thankyou
EDIT: Just re-read this and some of the spelling is really bad but cant be bothered to correct it as i think its pretty obvious what i mean.
Needing some help here please, in chemistry we have been determining the exact mass of iron in women's health iron tablets (Wilkos own). the bottle states each tablet contains 14mg of iron.
Steps: crush up 5 iron tablets and dissolve in sulfuric acid, I believe the concentration and amount is not important, it is just to stop the iron from oxidising.
make up a standard solution of 250cm^3 .... KMnO4/H+ 0.02M
when running through the following calculations it should titrate with 1.2cm^3 of Pottasium permangante solution (acidified) however the actual mean titre is 12cm^3, 10x more.
5 x 14mg = 70mg
This is what you are trying to find out ...
mol iron = 0.07/56 = 1.25 x 10-3 in 250ml solution
Concentration = 5 x 10-3 mol dm-3
How much of this standard solution did you titrate? 25ml ?
If so moles of iron = 1.25 x 10-4
This reacts with 2.5 x 10-5 mol manganate(VII) ions
If the molarity of the manganate ions = 0.02M, this represents a volume of (2.5 x 10-5)/0.02 = 0.0125 litres = 12.5 ml
(original mass - mass lost/original mass) x iron content = 67x10^-3 g 67x10^-3/56(RMM) = 1.196x10^-3 mol 1.196x10^-3 x 1000/250 = 4.786x10^-3 moldm^3 (iron solution) 4.786x10^-3 x (25/1000) = 1.1965x10^-4 mol (used for one titration) (5 iron ions wirth one manganate ion) 1.1965x10^-4 / 5 = 2.393x10^-5 mols 2.393x10^-5 / 0.02 = 1.1965x10^-3 dm^3 of KMnO4 solution dm^3 ---> cm^3 (x1000) 1.1965x10^-3 x 1000 = 1.1965 cm^3
Obviosuly wilkos arent going to bewrong on the mass of iron in each tablet (iron sulpahte i believe). so me the teacher and a friend in the class have ran through this calculation multiple times and still cannot find a mistake. the next logical thought was that the KMnO4/H+ solution was not actually 0.02M but 0.002M
So we have now titrated the KMnO4 solution with a solution of iron sulpahte we made up our selves. Mr of iron sulphate = 278
We made up a 0.1M solution of iron sulphate by dissolving+ 2.78g of iron sulphate into 100cm^3 solution.
The mean titre was 25cm^3
0.1 x (25/1000) = 2.5x10^-3 mol
same as before 1:5 reaction
2.5x10^-3 / 5 = 5x10^-4 mol in 25cm^3 therefore
1000/25 x 5x10^-4 = 0.02M KMnO4 solution.
I'm pretty much certain all the maths is correct (although not all the grammar and spelling is in this post) as 3 of us have checked it over multiple times. any suggestions to what could be going wrong? Thankyou
EDIT: Just re-read this and some of the spelling is really bad but cant be bothered to correct it as i think its pretty obvious what i mean.