YEAR 2 : Practical 1 - Simple redox titration

Question 1

Aim of this practical

Answer 1

To determine the relative molecular mass of an iron (II) salt by titration with standard potassium manganate (VII) solution

Question 2

Apparatus

Answer 2

3 decimal place mass balance
Safety goggles
A 50cm3 burette and funnel
25cm3 pipette and fille
250cm3 conical flasks
250cm3 volumetric flask

Question 3

Chemicals required

Answer 3

Unknown iron (II) salt
1moldm3 H2SO4 solution
Standardised KMnO4 solution (approx. 0.02moldm-3)

Question 4

Safety considerations

Answer 4

1moldm-3 H2SO4 solution —> irritant
KMnO4 solution —> harmful, oxidising, stand skin readily = gloves should be worn when handling the solid

Question 5

Method

Answer 5

1. Weigh out accurately, about 9.8g of the iron (II) salt provided and record the mass
2. Make the salt up to 250cm3 of a standard solution in H2SO4 solution
3. Titrate 25cm3 portions of this solution against the standardised KMnO4 solution
4. Use your results to calculate the relative molecular mass of the iron (II) salt

Question 6

When can we stop titrating?

Answer 6

When we have 2 concordant results

Question 7

Concordant results

Answer 7

Within +-0.20cm3 of each other

Question 8

How do we work out a titre?

Answer 8

Final volume - initial volume

Question 9

How do we work out exactly how much of the solid iron (II) salt is added?

Answer 9

1. Mass of solid
2. Mass of solid + container (add the solid to the beaker here)
3. Mass of container

Question 10

Full redox equation taking place

Answer 10

MnO4- +5Fe2+ + 8H+ —> Mn2+ + 5Fe3+ + 4H2O

Question 11

How do we work out the relative mass of the iron (II) salt?

Answer 11

1. Calculate mean titre volume
2. nMnO4 = c x v
3. nFe2+ (25cm3 pipetted) = stage 2 x 5
4. NFe3+ (250cm3 = entire volumetric flask) = stage 3 x 10
5. M = m/n