Hey guys!

Iron(II) ions in solution are oxidized by the air to iron(III) ions.

The amounts of iron(II) and iron(III) ions in solution can be found by using titrations

with acidified potassium manganate(VII) which only reacts with the iron(II) ions.

MnO4

−

+ 5Fe2+ + 8H+ ĺ Mn2+ + 5Fe3+ + 4H2O

• 25.0cm3

portions of a solution A, containing a mixture of iron(II) and iron(III)

ions, were acidified and titrated with potassium manganate(VII) solution of

concentration 0.0195mol dm−3.

• The mean titre was 16.80cm3

.

• About 150 cm3

of solution A was reacted with excess zinc, which reduced the

iron(III) ions to iron(II) ions. The excess zinc was filtered off.

• 25.0cm3

portions of this reduced solution, which contained iron(II) ions but no

iron(III) ions were acidified and titrated with potassium manganate(VII) solution

of concentration 0.0195 mol dm−3.

• The mean titre was 18.20cm3

.

Calculate the mass of iron(II) ions and the mass of iron(III) ions in 500cm3

of the

original solution A.

In this question,I got 4/5 marks.

But at the very end,my mass of Fe3+ comes 1.98g,but in the mark scheme it comes 0.152g.

I am not understanding why.

Please somone explain it!

Iron(II) ions in solution are oxidized by the air to iron(III) ions.

The amounts of iron(II) and iron(III) ions in solution can be found by using titrations

with acidified potassium manganate(VII) which only reacts with the iron(II) ions.

MnO4

−

+ 5Fe2+ + 8H+ ĺ Mn2+ + 5Fe3+ + 4H2O

• 25.0cm3

portions of a solution A, containing a mixture of iron(II) and iron(III)

ions, were acidified and titrated with potassium manganate(VII) solution of

concentration 0.0195mol dm−3.

• The mean titre was 16.80cm3

.

• About 150 cm3

of solution A was reacted with excess zinc, which reduced the

iron(III) ions to iron(II) ions. The excess zinc was filtered off.

• 25.0cm3

portions of this reduced solution, which contained iron(II) ions but no

iron(III) ions were acidified and titrated with potassium manganate(VII) solution

of concentration 0.0195 mol dm−3.

• The mean titre was 18.20cm3

.

Calculate the mass of iron(II) ions and the mass of iron(III) ions in 500cm3

of the

original solution A.

In this question,I got 4/5 marks.

But at the very end,my mass of Fe3+ comes 1.98g,but in the mark scheme it comes 0.152g.

I am not understanding why.

Please somone explain it!

I’ve highlighted in bold some important things I think you’ve missed.

The solution contains both Fe^2+ and Fe^3+ - the first titration tells you how much Fe^2+ there is and the second tells you how much Fe^2+ AND Fe^3+ there is.

The titre in the first titration (i.e volume of KMnO4 dropped from the burette) is 16.80 cm^3 and the concentration of KMnO4 is 0.0195 mol dm^-3.

The moles of KMnO4 = 16.80/1000 dm^3 x 0.0195 mol dm^-3 = 0.0003276 mol

But from the equation, we can see that for every KMnO4 used, 5Fe^2+ ions are consumed.

So moles of Fe^2+ = 5 x 0.0003276 mol = 0.001638 mol

But this is in just a 25 cm^3 portion - the total amount of solution is 500 cm^3, so there are actually (500 cm^3)/(25 cm^3) = 20 times the calculated amount of Fe^2+ in the entire solution:

Total mol Fe^2+ = 20 x 0.001638 mol = 0.03276 mol

We can now repeat the above procedure using a titre of 18.20 cm^3 instead of 16.80 cm^3 to find the total amount of both Fe^2+ and Fe^3+. Doing this shows that the total amount of iron in the solution is 0.03549 mol.

The amount of Fe^3+ is the total amount minus the amount of Fe^2+, so 0.03549 mol - 0.03276 mol = 0.00273 mol.

We can look up the relative mass of iron from the periodic table (55.8 g/mol) and we find that the mass of Fe^3+ is

0.00273 mol x 55.8 g/mol = 0.152 g (3 sf)

My suspicion is you just calculated the total moles of iron and then multiplied by 55.8 g/mol. This is the total mass of iron in the solution - i.e the mass of both Fe^2+ and Fe^3+.

The solution contains both Fe^2+ and Fe^3+ - the first titration tells you how much Fe^2+ there is and the second tells you how much Fe^2+ AND Fe^3+ there is.

The titre in the first titration (i.e volume of KMnO4 dropped from the burette) is 16.80 cm^3 and the concentration of KMnO4 is 0.0195 mol dm^-3.

The moles of KMnO4 = 16.80/1000 dm^3 x 0.0195 mol dm^-3 = 0.0003276 mol

But from the equation, we can see that for every KMnO4 used, 5Fe^2+ ions are consumed.

So moles of Fe^2+ = 5 x 0.0003276 mol = 0.001638 mol

But this is in just a 25 cm^3 portion - the total amount of solution is 500 cm^3, so there are actually (500 cm^3)/(25 cm^3) = 20 times the calculated amount of Fe^2+ in the entire solution:

Total mol Fe^2+ = 20 x 0.001638 mol = 0.03276 mol

We can now repeat the above procedure using a titre of 18.20 cm^3 instead of 16.80 cm^3 to find the total amount of both Fe^2+ and Fe^3+. Doing this shows that the total amount of iron in the solution is 0.03549 mol.

The amount of Fe^3+ is the total amount minus the amount of Fe^2+, so 0.03549 mol - 0.03276 mol = 0.00273 mol.

We can look up the relative mass of iron from the periodic table (55.8 g/mol) and we find that the mass of Fe^3+ is

0.00273 mol x 55.8 g/mol = 0.152 g (3 sf)

My suspicion is you just calculated the total moles of iron and then multiplied by 55.8 g/mol. This is the total mass of iron in the solution - i.e the mass of both Fe^2+ and Fe^3+.

- Using IAL/old spec edexcel papers as revision for A-Level edexcel Chemistry
- EDEXCEL IAL CHEM UNIT 5 - i think i failed
- Ever expected to get a B after an Edexcel A Level exam, but turned out you got an A?
- What units do I need for Edexcel IAL or IAS Further Maths
- How to Calculate Required UMS for an A* in EDEXCEL IAL Physics and Chemistry.
- How to Calculate Required UMS for an A* and A in EDEXCEL IAL Chemistry and Physics
- How to Calculate Required UMS for an A* and A in EDEXCEL IAL Chemistry and Physics
- Edexcel IAL BIOLOGY JANUARY 2024
- Edexcel IAL Business Studies Notes
- Edexcel ial information technology
- Edexcel A Level Chemistry Paper 2 Tuesday 18th June 2024 9CH0/02
- IAL Biology U5 Scientific article
- IAL A level subjects- after year1- adding in Psycholgy
- IAL Edexcel recent past papers
- Chemsitry A level help
- chemistry alevel mock
- U grade in cash in
- accounting notes
- Do colleges care about what modules I took for my IAL maths and further maths?
- A-level physics suggestions

Latest

Trending

Last reply 4 weeks ago

Determining how the temperature affects the pH of milk using titrationLast reply 1 month ago

How can I use 3D model kit to support my learning in A-level chemistry?