**Aim**

The purpose of this experiment is to determine x in the formula Fe(NH_{4})_{2}(SO_{4})_{2•}xH_{2}O by titration against a standard solution of potassium manganate (VII) (permanganate).

**Introduction**

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(NH_{4})_{2}(SO_{4})_{2•}xH_{2}O, the precise concentration of which (in g dm^{-3}) is given by the teacher.

B is a solution of potassium manganate(VII) (permanganate), KMnO_{4}, the precise concentration of which (in mol dm^{-3}) is given by the teacher.

**Requirements**

- safety glasses
- pipette, 25 cm
^{3} - pipette filler
- 3 conical flasks, 250 cm
^{3} - ammonium iron(II) Sulfate solution, A (IRRITANT)
- sulfuric acid, dilute, 1 M H
_{2}SO_{4} - burette, 50 cm
^{3} - funnel, small
- potassium manganate (VII) solution, B (0.01 mol dm-3) (LOW HAZARD)
- white tile
- wash-bottle of distilled water

**Procedure**

Pipette 25 cm^{3} 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:

MnO_{4}^{–} _{(aq)} + 5Fe^{2+} _{(aq)} + 8H^{+} _{(aq)} → Mn^{2+} _{(aq)} + 5Fe^{3+} _{(aq)} + 4H_{2}O _{(l)}

**Calculation**

Use your results to determine x in the formula Fe(NH_{4})_{2}(SO_{4})_{2•}xH_{2}O. 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.)

**Calculation steps**

- From the titre and the equation for the reaction calculate the concentration of Fe
^{2+}ions (*y*mol dm^{-3}). - From the concentration calculate the mass of anhydrous Fe(NH
_{4})_{2}(SO_{4})_{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.