IB Colourful Solutions in Chemistry

Water of crystallisation means that water molecules have been incorporated into an ionic lattice. This usually happens in stoichiometric ratios between the water molecules and the ionic compound.

Note: Titrations work better with solution concentrations of between 0.01 and 0.2 mol dm^-3.

Background
Requirements
Procedure
Safety

Background

Potassium manganate(VII) is a powerful oxidising agent which reacts in acidic solution with iron(II) compounds producing iron(III) ions. This, allied to the deep purple colour of the manganate(VII) ion, allows potassium manganate(VII) to be used as a self-indicating titrant for the analysis of iron salts.

manganate(VII) ions reacting with iron(II) ions

5Fe²⁺(aq) + MnO₄^-(aq) + 8H⁺ → 5Fe³⁺(aq) + Mn²⁺(aq) + 4H₂O(l)

On the reactants, the manganate(VII) ions have an intense purple colour, while on the right hand side iron(III) ions are pale yellow and manganese(II) ions are pale pink in solution. At the concentrations used, all of the products are essentially colourless.

In this experiment, a known mass of a hydrated ammonium iron(II) sulfate is analysed for iron(II) content and the results used to determine the number of water of crystallisation molecules per ammonium iron(II) sulfate formula units.

The calculation requires that the students understand the meaning of a chemical formula.

Hydrated ammonium iron(II) sulfate

(NH₄)₂Fe(SO₄)₂.xH₂O.

Every formula unit of the salt contains two ammonium ions, one iron(II) ion and two sulfate ions, with an unknown number of water molecules.

This ratio is maintained regardless of the amount of the salt, i.e. one mol of iron to 2 mol of ammonium ions, etc..

Hence, if we can determine the mol of iron(II) ions, we also know the mol of the salt and, from the relative masses of the atoms, its mass.

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Requirements

Chemicals

Potassium manganate(VII) (aq), 0.02 mol dm^-3
Hydrated Ammonium iron(II) sulfate crystals
Sulfuric acid, 2M

Apparatus

Burette, 50ml
Pipette, 25ml
Volumetric flask, 250ml
Clamp, boss and stand
White tile

Procedure

Accurately weigh out approximately 10g of the ammonium iron(II) sulfate (hydrated).
Dissolve the salt in 100 cm³ of 1M sulfuric acid in a volumetric flask and make up to the mark with water.
Transfer 25ml of the ammonium iron(II) sulfate solution into a conical flask and titrate against the potassium manganate(VII) solution until the first permanent pink colour.
Repeat the titration until concordant results are obtained.

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Recording the data

Ensure that all of the data is recorded with suitable units and inaccuracies.

The following shows example raw data and analysis.

Raw data

Mass of potassium hydrogenphthalate = 5.22g ± 0.01 (here we use the actual mass weighed out, as an example)

The initial solutions are all colourless.

Titre	Initial burette reading /ml ± 0.05	Final burette reading /ml ± 0.05	Volume added /ml ± 0.1	Observations
1	0.00	25.25	25.25	red end
2	0.05	24.85	24.80	pink end
3	0.00	24.75	24.75	pale pink

Data analysis

Titres 2 & 3 are concordant and were used to average the titration volume = (24.80 + 24.75)/2= 24.775

This is rounded to three sigificant figures as the inaccuracy is ± 0.1

Hence titre = 24.8 ml ± 0.1

The moles of potassium hydrogenphthalate used = mass/Mr = 5.22/204.23 = 0.00257

Molarity of the potassium hydrogenphthalate solution used = 0.00256/0.25 = 0.102 mol dm^-3

Moles of potassium hydrogenphthalate in 25 ml aliquot = 0.102 x 0.025 = 2.56 x 10^-3

Equation for the reaction between sodium hydroxide and potassium hydrogenphthalate (abbreviated to KHPh):

NaOH + KHPh KNaPh + H₂O

Hence moles of sodium hydroxide also = 2.56 x 10^-3

Volume of sodium hydroxide from the titration = 24.8 ml

Hence molarity of sodium hydroxide = 0.103

The was diluted in a ratio of 1:10 from the original stock bottle

Hence concentration of sodium hydroxide in the original stock bottle = 1.03 mol dm^-3

Treatment of errors and inaccuracies

These should be recorded as percentage inaccuracy and then propagated through a typical series of steps:

1 Making the potassium hydrogenphthalate solution:

Mass of potassium hydrogenphthalate = 5.22g ± 0.01 = 0.19% (inaccuracy)

Volumetric flask 250 ml ± 0.23 = 0.092%

2 Transfering 25 ml aliquot (pipette)

25 ml ± 0.06 = 0.24%

3 Titration inaccuracy

24.8 ml ± 0.1 = 0.40%

Total inaccuracy = 0.19 + 0.09 + 0.24 + 0.40 = 0.92%

Applying this inaccuracy to the sodium hydroxide give molarity = 1.03 ± 0.01

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Safety

At the concentrations used in this experiment, none of the components are hazardous except sulfuric acid (1M), which is corrosive and spillages must be cleaned with plenty of water.

Safety glasses must be worn.