Example 2
• In order to determine the concentration of a commercial
sample of hydrogen peroxide (H2O2
), 15 mL of it was
diluted to 300 mL. Then 20 mL of diluted solution was
acidified and titrated with a 0.021 M potassium
permanganate solution (KMnO4
). An average volume of
18.72 mL was required. Determine the concentration of
the original hydrogen peroxide.
• Combining the two half equations we get:
• 2MnO4
- + 5H2O2 + 6H+ → 2Mn2+ + 8H2O + 5O2
• KMnO4
(Titrant) c = 0.021 M, V = 18.72 mL
• H2O2
(Analyte) c = ?, V = 20 mL
• nKMnO4 = c x V = 0.021 M x 0.01872 L (convert mL to L)
• = 3.9312 x 10-4 mol
• 5 5
• nH2O2 = ---- x n(KMnO4
) = --- x 3.9312 x 10-4 mol
2 2
= 9.828 x 10-4 mol
n 9.828 x 10-4 mol
cH2O2 = --- = ------------------------ = 0.0491M
V 0.02 L
• c(diluted H2O2
) = 0.0491 M
• Since the original solution was diluted 20 times (i.e. 300
mL ÷ 15 mL = 20 dilution factor) we multiply by 20:
• c (undiluted/original H2O2
) = 0.982 M
• Note: The end-point of this reaction is indicated by a
slight pink colour (due to an excess of MnO4
-ion).
explain the process of how they get the answers
1. The first step was to calculate the amount of potassium permanganate (KMnO4) used in the titration. This was done by multiplying the concentration of KMnO4 (0.021 M) by the volume used (18.72 mL converted to L).
2. Next, the moles of hydrogen peroxide (H2O2) were calculated using the stoichiometry of the reaction between KMnO4 and H2O2. Because the balanced equation shows a 1:5 ratio of KMnO4 to H2O2, the moles of H2O2 were determined to be five times the moles of KMnO4.
3. The concentration of the original H2O2 solution was then calculated by dividing the moles of H2O2 by the volume used in the titration (20 mL converted to L).
4. Since the original H2O2 solution was diluted 20 times, the concentration was multiplied by 20 to determine the concentration of the undiluted/original H2O2 solution.
5. The pink color change observed during the titration indicated the endpoint of the reaction, showing that the titration was complete.
Overall, the process involved stoichiometry calculations based on the balanced chemical equation of the reaction, as well as conversion of units for volume and concentration. By following these steps, the concentration of the original hydrogen peroxide solution was successfully determined.