H_{2}O_{2}Decomposition Lab II(Reference: Chen, C.,Ehrenkranz, D., Sconzo, P., Chemistry in Microscale: Advanced Laboratories, 11th Biennial Conference on Chemical Education, Georgia Tech University, 1990.)

PREPARATIONPolyethelene pipets – 2 per group

6 inch test tubes – 1 per group

Balances – 1 per group

Fresh bottle of hydrogen peroxide (3%)

50 mL 6 M H_{2}SO_{4}(at reagent bench or 10 mL per group)

KMnO_{4 }solution moles solute/ gram solution – NOT MOLARITY or MOLALITY

· Mass 150 mL beaker

· Add ~ 0.7 g KMnO_{4}

· Add 100 mL water

· Mass beaker again

· Record concentration as moles of solute/ gram of solution

BACKGROUNDIn this experiment we want to determine how much hydrogen peroxide (H

_{2}O_{2}) is in a store-bought solution of H_{2}O_{2}, because in the SOLUTION there will be some H_{2}O_{2}and the rest is mostly water. In order to do this we will oxidize the H_{2}O_{2}using potassium permanganate (KMnO_{4})- this is an example of an oxidation/reduction reaction, or REDOX reaction. You will be able to determine when the decomposition is complete because the solution in the test tube will turn from clear and colorless to clear and pink (presence of the Mn_{2}^{+}ions that you see on the PRODUCTS side of the reaction give the pink color). You will have the mass of both of the reactants, H_{2}O_{2}and KMnO_{4}, and you know the concentration of the KMnO_{4}solution so you can calculate the mass of H_{2}O_{2}reacted. Once you have that you can calculate the mass percent of H_{2}O_{2}in the H_{2}O_{2}solution.

2MnO_{4}^{-}+ 6H^{+}+ 5H_{2}O_{2}8H_{2}O + 5O_{2}+ 2Mn^{2+}The concentration units are like MOLARITY, except the units are number of moles of solute per kilogram of solution (mol/kg). Remember that molarity units are number of moles of solute per liter of solution. These units are convenient because you to prepare a certain concentration of solution by mass, and since you are keeping track of the masses of the reactants during your experiment these units for the reactants will be more convenient for calculations.

PROCEDURE1. Label, and then fill a pipet with H

_{2}O_{2}solution2. Label, and fill a second pipet with KMnO

_{4}solution3. Mass both pipets and record

4. Add 7 drops of the H

_{2}O_{2}solution to a small test tube5. Add 3 drops of 6M H

_{2}SO_{4}(CAUTION!!)6. Add KMnO

_{4}solution dropwise, mixing after each addition, until a faint pink color persists7. Re-mass each pipet and record

8. Using same pipets do a second trial

9. Clean up!

DATA

o | TRIAL I | TRIAL II |

Concentration of KMnO_{4}, mol/kg |
0 | 0 |

Concentration of KMnO_{4}, mol/g |
0 | 0 |

Initial Mass of H_{2}O_{2 }pipet, g |
0 | 0 |

Final Mass of H_{2}O_{2 }pipet, g |
0 | 0 |

Initial Mass of KMnO_{4}, g |
0 | 0 |

Final Mass of KMnO_{4}, g |
0 | 0 |

CALCULATIONS

1. Determine the number of MOLES of KMnO_{4}reacted

2. What is the MOLE to MOLE relationship between H

_{2}O_{2}and KMnO_{4}

3. Determine the number of grams of H

_{2}O_{2}reacted

4. Determine the MASS PERCENT of H

_{2}O_{2}in the H_{2}O_{2}solution

5. If the mass percent of commercial H

_{2}O_{2}is 3.0%, determine you PERCENT ERROR

Practice Calculations for BEFORE the LabYou should read the lab before you do these calculations so that you will understand the procedure for determining each value.

Suppose you performed the experiment as described, you used 25.0g of H

_{2}O_{2}solution and 2.18g KMnO_{4}solution. The concentration of your KMnO_{4}solution is 4.0 x 10–8 mol solute/kg solution. Complete the following calculations to find MASS PERCENT of the H_{2}O_{2}solution.2MnO

_{4}^{-}+ 6H^{+}+ 5H_{2}O_{2}8H_{2}O + 5O_{2}+ 2Mn^{2+}

1. Find mol/g from mol/kg.

2. Determine the number of moles of KMnO

_{4}that reacted.

3. From the balanced reaction, determine the mole to mole ratio of H

_{2}O_{2}to KMnO_{4}.

4. Determine the number of grams of H

_{2}O_{2}consumed in the reaction. (Hint: H_{2}O_{2}pure, not grams of the solution, you already know that from the question).

5. Now determine the grams of H

_{2}O_{2}per gram of H_{2}O_{2}solution then multiply by 100, this is the mass percent of H_{2}O_{2}in the solution.

6. Determine the percent error in this experiment if the mass percent on the bottle of H

_{2}O_{2}solution read 3.1%. For percent error you subtract the actual from the experimental and then divide by the actual, then multiply by 100.

ANSWERS to Practice CalculationsYou should read the lab before you do these calculations so that you will understand the procedure for determining each value.

Suppose you performed the experiment as described, you used 25.0g of H

_{2}O_{2}solution and 2.18g KMnO_{4}solution. The concentration of your KMnO_{4}solution is 4.0 x 10–8 mol solute/kg solution. Perform the following calculations to find MASS PERCENT of the H_{2}O_{2}solution.2MnO

_{4}^{-}+ 6H^{+}+ 5H_{2}O_{2}8H_{2}O + 5O_{2}+ 2Mn^{2+}1. Find mol/g from mol/kg.

(4.0 x 10–8 mol/kg) x (1000 g/kg) =

4.0 x 10–5 mol/g KMnO_{4}

2. Determine the number of moles of KMnO

_{4}that reacted.

(4.0 x 10–5 mol/g KMnO

_{4}) x (2.18 g KMnO_{4}) =8.72 x 10-5 mol KMnO_{4}

3. From the balanced reaction, determine the mole to mole ratio of H

_{2}O_{2}to KMnO_{4}.

5 mol H_{2}O_{2}to 2 mol KMnO_{4}

4. Determine the number of grams of H

_{2}O_{2}consumed in the reaction. (Hint: H_{2}O_{2}pure, not grams of the solution, you already know that from the question).

(8.72 x 10-5 mol KMnO

_{4}) x (5 mol H_{2}O_{2}/ 2 mol KMnO_{4}) x (34 g/mol H_{2}O_{2}) =7.4 x 10-3 g H_{2}O_{2}

5. Now determine the grams of H

_{2}O_{2}per gram of H_{2}O_{2}solution then multiply by 100, this is the mass percent of H_{2}O_{2}in the solution.

(7.4 x 10-3 g H

_{2}O_{2}) / (25.0 g H_{2}O_{2}solution) x 100 =3.0 % by mass H_{2}O_{2}

6. Determine the percent error in this experiment if the mass percent on the bottle of H

_{2}O_{2}solution read 3.1%. For percent error you subtract the actual from the experimental and then divide by the actual, then multiply by 100.(3.1 –3.0) / (3.1) x 100 =

~3 % error

DISPOSAL· Fresh KMnO

_{4}should be made each year, old can be reacted with hydrogen peroxide

· Extra 6 M sulfuric acid can be saved

· Extra hydrogen peroxide should be tightly capped and kept in the dark

· Fresh yeast should be purchased each year

· Student solutions can all go down the drain

Ginger Chateauneuf, 2000.

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