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Finally, dilute sulphuric acid was added to the flask until the solution was about 1cm below the graduation mark. Once this was the case, the acid was added slowly from a clean dropping pipette until the bottom of the meniscus of the liquid was just touching the graduation line. The flask was then stoppered and the flask was inverted several times to homogenate the solution.
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A pipette and pipette filler were used to withdraw 25cm3 of the solution from the volumetric flask and transferred to a conical flask. (Process 1)
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Potassium Manganate (VII) was then added to a clean 100cm3 beaker and transferred to a clean burette. A small amount of the solution was allowed to run through the jet to make sure it was full of solution. (Process 2)
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The volume reading on the burette before titration was recorded in a table to the nearest 0.05cm3. (Process 3)
- Potassium Manganate was then added in small volumes to the solution in the conical flask, and after each addition the flask was swirled. The purple colour of the Manganate ions disappeared as they reacted with the iron ions. When a faint purple colour began to emerge in the solution this was the end point of the titration. (Process 4)
- The final burette reading was recorded and the volume of solution run out into the flask was calculated. (Process 5)
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Processes 1,2,3,4 and 5 were repeated until at least three titration volumes agreed to within 0.1cm3
Raw Results:
Mass of weighing bottle and solid 6.85g
Mass of weighing bottle 1.93g
Mass of solid 4.92g
Results
Titration rough 1 2 3 4 5
Final burette 25.85 26.10 26.40 25.85 25.80 25.75
Reading
Initial burette 0.55 0.90 0.80 0.85 0.85 0.75
Reading
Titre 25.30 25.20 25.60 25.00 24.95 25.00
11. Average titre = 24.98cm3
12.
13. 250cm3/titre = n
70n = amount of Fe
250/24.98 = 10.008
70mg x 10.008 = 700.56mg
14. 70mg
15. 0.70/4.92 = 0.14227
ans x 100 = practical percentage of iron in iron (II) ammonium sulphate crystals
therefore the practical percentage = 14.23%
16a. (NH4)2SO4FeSO4.6H2O
16b. Mr
2 x N = 28
20 x H = 20
2 x S = 64
14 x O = 224
1 x Fe = 55.8
Mr = 391.8 Theoretical percentage = Ar of Fe/Mr of compound
= 55.80/391.80
= 0.1424
% = 0.1424 x 100
= 14.24%
Theoretical percentage = 14.24%
Practical Percentage = 14.23%
This means that this particular Iron (II) ammonium sulphate compound was quite pure (only a small amount of the Iron (II) in the compound has been oxidised).
Evaluation:
Percentage error = error x 100
reading
Quantity measured % error
Mass of iron compound weighed on balance 0.1
250cm3 solution made up in volumetric flask 0.08
25cm3 solution delivered by pipette 0.24
average titre 0.2
Percentage error of graph: 0.07% (5/70)
Error Identification-
The first stage in the procedure, which could have led to errors, is transferring the iron (II) ammonium sulphate to the clean beaker. This would have led to a lower titration result. The second stage in the procedure, which could have led to errors, is the transfer of the solution to the volumetric flask. This would also have led to a lower titration result. The third stage in the procedure, which could have led to errors, is the dropping of potassium manganate (VII) solution into the iron (II) solution. This error would have led to a higher titration result.
The final error, which was identified, would have the most impact on the overall titration result.
James Holman
