The purpose of this experiment is to determine x in the formula Fe(NH4)2(SO4)2•xH2O by titration against a standard solution of potassium manganate (VII) (permanganate).
This experiment provides useful experience in preparation for a practical examination. The directions are similar to those that would be given by an examination board.
A is a solution of ammonium iron(II)sulfate, Fe(NH4)2(SO4)2•xH2O, the precise concentration of which (in g dm-3) is given by the teacher.
B is a solution of potassium manganate(VII) (permanganate), KMnO4, the precise concentration of which (in mol dm-3) is given by the teacher.
- safety glasses
- pipette, 25 cm3
- pipette filler
- 3 conical flasks, 250 cm3
- ammonium iron(II) Sulfate solution, A (IRRITANT)
- sulfuric acid, dilute, 1 M H2SO4
- burette, 50 cm3
- funnel, small
- potassium manganate (VII) solution, B (0.01 mol dm-3) (LOW HAZARD)
- white tile
- wash-bottle of distilled water
Pipette 25 cm3 of the ammonium iron(II) Sulfate solution, A, into a conical flask and add an equal volume of dilute sulfuric acid. Titrate with potassium manganate(VII) solution, B, until a permanent faint pink colour appears. Repeat the titration twice and enter your results in a copy of your results table.
The overall equation for the reaction is:
MnO4– (aq) + 5Fe2+ (aq) + 8H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
Use your results to determine x in the formula Fe(NH4)2(SO4)2•xH2O. You should set out your calculations so that every step in your working is clearly shown. If you cannot work out a method of calculation, use the suggestions below. (These would probably not be given in an examination.)
- From the titre and the equation for the reaction calculate the concentration of Fe2+ ions (y mol dm-3).
- From the concentration calculate the mass of anhydrous Fe(NH4)2(SO4)2 in one litre of solution.
- Subtract the mass obtained in step 2 from the mass of the salt in one litre. This difference, z g, divided by 18 g mol-1 gives the amount of water of crystallization in y mol of the salt.
- x mol is the amount of water in 1 mol of the salt.