Date: …………….
Page No. …..
Experiment No. 3
Name of Experiment: To find out percentage purity of a sample of hydrated ferrous sulphate FeSO4.7H2O,
31 g of which have been dissolved in 1 L of solution. M/50 solution of potassium permanganate is provided.
Apparatus Required: Beaker, burette, pipette, conical flask, stand, funnel.
Chemicals Required: Solution of ferrous sulphate, solution of KMnO4, sulphuric acid.
Principle:
Potassium permanganate oxidises ferrous ion of ferrous sulphate to ferric ion in acidic
medium according to following ionic equation:
𝑀𝑛𝑂4−
+
8 𝐻+
+
5e
𝐹𝑒 2+
→
→
𝑀𝑛2+
+
𝐹𝑒 3+
+
4 𝐻2 𝑂
e
}x5
--------------------------------------------------------------------------------------------------------------𝑀𝑛𝑂4−
+ 5 𝐹𝑒 2+
+
8 𝐻+ →
𝑀𝑛2+
+
5𝐹𝑒 3+ + 4 𝐻2 𝑂
Procedure:
Burette, pipette, conical flask and funnel were washed properly. The pipette was rinsed
with ferrous sulphate solution. 20 mL of ferrous sulphate solution was taken in the conical flask with help of
pipette. One test tube of dilute sulphuric solution was added to it.
The burette was rinsed with given solution of potassium permanganate and then filled with
this solution. Initial reading was noted. Now from the burette potassium permanganate solution was added
to conical flask slowly and drop wise with regular shaking till the permanent pink colour was imparted to the
solution. Final reading of burette was noted. Difference was calculated.
The steps were repeated 4-5 times to get a set of three concordant readings.
Observations:
Volume of ferrous sulphate solution taken for each titration = 20 mL
S. No.
Initial Burette reading
Final Burette reading
1
5 mL
25.2 mL
Volume of KMnO4 solution
used
20.5 mL
2
20.5 mL
40.5 mL
20 mL
3
8 mL
28 mL
20 mL
4
28 mL
48 mL
20 mL
Concordant volume = 20 ml
Colour change at end point = Colourless to pink (Permanganate solution was taken in the burette)
Indicator used = KMnO4 acts as self-indicator
Calculations:
𝑀𝐾𝑀𝑛𝑂4 . 𝑉𝐾𝑀𝑛𝑂4
𝑀𝐹𝑒𝑆𝑂4 . 𝑉𝐹𝑒𝑆𝑂4
=
1
5
𝑀𝐾𝑀𝑛𝑂4
= Molarity of 𝐾𝑀𝑛𝑂4 = M/50 = 0.02 M
𝑉𝐾𝑀𝑛𝑂4
= Volume of 𝐾𝑀𝑛𝑂4 = 20 mL
0.02 × 20
𝑀 𝐹𝑒𝑆𝑂4
× 20
=
𝑀 𝐹𝑒𝑆𝑂4 = Molarity of ferrous sulphate = ?
𝑉𝐹𝑒𝑆𝑂4 = Volume of ferrous sulphate = 20 mL
1
5
𝑀 𝐹𝑒𝑆𝑂4 = 0.02 x 5
=
0.1 M
Strength (g/ L) = Molarity x molecular mass = 0.1 x 278 = 27.8 g/L
Percentage purity of the sample =
27.8
31
x 100 = 89.67 %
Precautions:
1. The conical flask should not be rinsed.
2. Reading of upper meniscus of permanganate solution should be noted.
3. Permanganate solution should be added to conical flask only drop wise with regular shaking.
4. Correctness of end point should be checked.