Pre-Lab Calculations

***Here, we are operating under the assumption that our sample will be 3% H2O2 , the density is 1 g/mL, and our potassium permanganate solution is 0.02M. This will then decide the numbers we use in our procedure. 

First, the reaction that will be taking place is as follows: 6H+ + 2MnO4- + 5H2O2 → 5O2  + 2Mn2+ + 8H2O

This is important to establish so that we have a mole ratio for calculations later. 

First, we can find the mass of H2O2 in a 1 mL sample like so:  1.00g/mL x 1mL x 0.03 = 0.03g H2O2

From there, we can calculate the moles of H2O2: 0.03g x 1 mol H2O2/34.016g H2O2 = 8.82 x 10-4  mols H2O2

Then, we can estimate approximately how much potassium permanganate will be needed to titrate the sample: 

8.82 x 10-4  mol H2O2 x 2 mol MnO4- / 5 mol H2O2 = 3.53 x 10-4 mol MnO4     ---> 3.53 x 10-4 mol x L/0.02 mol x 1000 mL/L = 20 mL 

So, if the sample of hydrogen peroxide is truly 3% and has not decomposed, it should take 20 mL of potassium permanganate to titrate 1 mL of the hydrogen peroxide. 

Post-Lab Calculations 

***The sample data we will be using is as follows: We titrated a 2 mL sample of hydrogen peroxide, and we ended up using only 10 mL of potassium permanganate. 

First, we will calculate the moles of potassium permanganate used: 10 mL MnO4 x 0.02 mols/L MnO4 x 1 L/1000 mL = 2 x 10-4 mols 

Then we can find the moles of hydrogen peroxide using the mole ratio: 2 x 10-4 mols MnO4 x 5 mols H2O2 / 2 mols MnO4 = 5 x 10-4 mols 

From there, the concentration of hydrogen peroxide in mol/L can be found:  5 x 10-4 mols / 2 mL H2O2  x 1000 mL / 1 L = 0.25 mol/L

Finally, we can find the concentration in percent by mass: 0.25 mol/L x 34.02 g / 1 mol x 1 L / 1000 mL x 100% = 0.85% 

So, the sample of hydrogen peroxide is actually only 0.85% rather than the 3% we had anticipated (Sounds like it's time to get a new bottle!)