Uncertainty for calculating the amount of KMnO4
Adding absolute uncertainty
44.5ml±0.0750ml – 23.0ml±0.075 = (45.4ml – 23ml) ± (0.0750ml + 0.0750ml)
= 21.50ml ±0.15ml
Table 5. Processed Data – Average of 1st Titration and 2nd Titration
Uncertainty for calculating the average amount of moles for NaC2O4 in 1st titration
Dividing absolute uncertainty
(0.30g±0.02g) / (23+23+16+16+16+12+12) = (0.30g±0.067%)/ (23+23+16+16+16+12+12)
= 0.30/134 ± 0.067%+0%
= 0.0022 ± 0.0067%
= 0.0022mol ± 1.8x10-7mol
Adding absolute and dividing absolute uncertainty
0.0022±1.8x10-7mol + 0.0023±1.8x10-7mol + 0.0023±1.8x10-7mol + 0.0022±1.8x10-7mol + 0.0025±1.8x10-7mol + 0.0026±1.8x10-7mol
= (0.0022 + 0.0023 + 0.0023 + 0.0022 + 0.0025 + 0.0026) ± (1.8x10-7mol x 6)
= 0.0141 ± 1.08x10-6mol
= 0.0141 ± 6x10-5%
(0.0141 ± 6x10-5%) / 6 = (0.0141 / 6) ± (6x10-5%+0)
= 0.0024 ± 6x10-5%
= 0.0024mols ± 1.44x10-8mols
Uncertainty for calculating the average amount of KMnO4 in 2nd titration
Adding absolute and dividing absolute uncertainty
20.6±0.15ml + 20.8±0.15ml + 20.6±0.15ml = (20.6 + 20.8 + 20.6) ± (0.15+0.15+0.15)
= 62.0ml ± 0.45ml
(62.0±0.45ml) / 3 = (62.0ml±0.73%) / (3±0%)
= (62.0/3) ± (0.73% + 0%)
= 20.7 ± 0.73%
= 20.7ml ± 0.15ml
Table 6. Summary Data of Processed Calculation
Calculation:
Form the half equation given above in the background information; it is possible to write down a balance equation for the reaction of oxalate ion with the permanganate ion. In this equation, sodium and potassium is not take part in the equation because they are spectators.
Hence, using the balanced equation, it is possible to determine the concentration of the potassium permanganate ion.
Again, from the half equation given above the background information; it is possible to write down a balance equation of hydrogen peroxide with the permanganate ion. Again, potassium in potassium permanganate does not take part in this equation because it is a spectator.
With the balanced equation, it is able to calculate the number of grams of hydrogen peroxide in the unknown sample. Also using this, percentage of hydrogen peroxide could be determined by the mass assuming the density of the hydrogen peroxide solution to be 1.00g/ml.
g
Conclusion and Evaluation:
In this titration experiment, potassium permanganate was used as an indicator for both of the titration involving sodium oxalate and hydrogen peroxide. In addition, sulfuric acid was used as a provider of hydrogen ion to the solution.
In case where if there is an insufficient amount of acid is added in either titration, with sodium oxalate or with hydrogen peroxide, some of the permanganate ions will be reduced to MnO2 instead of Mn2+. If the half equations are rewritten by replacing Mn2+ by MnO2;
Comparing with the previous half equations with Mn2+;
It can be seen that in the 1st titration, the molar ratio between MnO4- with C2O42- in the titration with Mn2+ is 2:5, whereas the molar ratio between MnO4- with C2O42- in the titration with MnO2 is 1:2.
Also, in the 2nd titration, the molar ratio between MnO4- with H2O2 in the titration with Mn2+ is 2:5, whereas the molar ratio between MnO4- with H2O2 in the titration with MnO2 is 2:1.
Therefore, it is clearly visible that in the process where there is an insufficient amount of acid in the solution reduces the permanganate ions to MnO2 instead of Mn2+, changes the molar ratio in both of the titrations.
Insufficient amount of acid will affect the volume of potassium permanganate to reach the end point. Since the acid is the limiting agent, the amount of potassium permanganate to react with the acid will be less. Therefore, insufficient amount of acid affects the volume of potassium permanganate to decrease to reach the end point.
In the second titration, dealing with hydrogen peroxide, sulfuric acid and potassium permanganate, the solution of hydrogen peroxide and sulfuric acid was to be heated. This process of heating shifts in endothermic direction allowing equilibrium of the reaction to move towards the right. This allows the percentage of hydrogen peroxide to react more to become oxygen and water. This is different from conducting the same experiment at room temperature. If the reaction was to take place at room temperature, then the percentage of hydrogen peroxide to react will be less than the experiment with heat added to the solution.
The reason why oxalic acid cannot be used in the standardization of potassium permanganate is because sodium oxalate is not hygroscopic. Hygroscopic is the ability of a substance to attract water molecules (H2O) from surrounding environment. Because sodium oxalate is not hygroscopic, it does not dissolve readily; hence it needs to be heated in order to carry out the experiment. Also this heating up the solution increases the rate of the reaction.
Hydrogen peroxide undergoes a reduction and oxidation reaction producing oxygen and water. This half equation of the process in reduction and oxidation reaction is called disproportionation. Disproportionation is a chemical reaction in which a reaction is simultaneously reduced and also oxidized to form two different products.
These two half equations are;
The theoretical value of the concentration of hydrogen peroxide was 0.35% (Dr Badran), as provided from teacher. The value gained from the experiments was 0.34% with the uncertainty of 0.015%. There is only 0.01% difference between the theoretical value and the gained value from the experiment.
Therefore, the percentage difference between the theoretical value and the gained value is;
(theoretical value – gained value) / (theoretical value) x 100
= (0.35% - 0.34%) / (0.35%) x 100
= (0.01 / 0.35) x 100
= 0.02857 x 100
= 2.86%
This 2.86% difference in the concentration of hydrogen peroxide may be able to become 0% by factors such as; repeating more trials of both titrations to reduce uncertainties and reduce uncertainties in the process of the experiment in reading and blocking out the heat and making the solution in constant temperature in the 1st titration.
Improving the Investigation:
Table 5. Weaknesses and Limitations
Table 6. Improvements