If we remind in chapter 7, one of the acid - base titrations we executed, was the determination of the percentage purity of a substance. In the demonstration of a alike applicability to redox reactions, we shall in this practical laboratory class attempt to find out the percentage purity of a reductant, FeSO4.7H7O. This we will do via titrating against the standardized solution of potassium permanganate.
Determination of the concentrations of a solution of FeSO4.7H2O using a standard solution of potassium permanganate.
Conical flask (25cm3)
1. Rinse the burette (50.0cm3) twice with a few cm3 of KMnO4 solution. Fill the burette with the KMnO4 solution above the Zero mark and drain to this mark, making sure the burette tip is full and any air is ejected within the body of the KMnO4 solution.
2. Pipette out 25.0cm3 of the solution of FeSO4.7H20 into a 250cm3conical flask and add to it 20cm3 of 2moldm-3 sulphuric acid. Heat to between 60 and 80°c.
3. Titrate the hot mixture through KMnO4 from the burette until the solution in the conical flask attains a permanent faint pink colour.
4. Recur the titration using fresh samples each time until at least 2 titrations are within 0.2cm3 accuracy.
5 Record your titre values using the format in earlier experiments and discover the average titre value.
6 From us consequences, determine
a The number of moles of potassium permanganate utilized in the solution.
b The number of moles of the iron compound.
c The concentration in moldm-3 of the iron compound
d The percentage purity of the crystals.
(Mn = 55, K = 39, 0 =16, C=12, H = 1, N =14, 5
S = 32, Fe =56)
Volume of KMnO4
Treatment of Results
Step 1: The equation of the reaction
MNO-4(aq) + Fe2+ (aq) + 8H (aq) Mn 2+ (aq ) + 4H2O
Step 2. We shall assume the followings:
1. The permanganate solution used is the standardized solution from the earlier experiment.
2. The amount in moldm-3 of the KMnO4 is 0.002 moldm-3.
3. The average titre value for KMnO4 is Wcm3 .
4. The FeSO4.7H2O solution was prepared in dissolving about 7.6g of the crystals in 250cm3 of solution.
Step3. The number of moles MnO4 = Molarity x (volume).W/ 1000
=W x 0.02 /1000
= M moles
Step4. Find the number of moles of the iron II crystals from the stoichimetric equation of the reaction
1 mole MnO4.5 moles Fe2+(aq )
M mole MnO4 = 5 x M mole Fe2+ (aq)
= 5 M mole in 25cm3
Step 5.The number in moles dm-3 of the Fe2+
= No of mole x 1000 / Volume
= 5M x 1000 / 25
= 200M molesdm-3
Step6. The mass Concentration of the FeSO4.7H2O
Mass concentration = moldin-3 x molar Mass
= 20M x 278gdm-3
Step7. Estimate the percentage purity of the iron salt
Percent purity = mass determined during experiment / Actual mass of iron in 100cm3
= Ngdm-3 x 100 / 4 x 7.6
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