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Homework Help: Stoichiometry problems thread

  1. Jul 5, 2011 #1
    Stoichiometry problem

    1. The problem statement, all variables and given/known data
    1 g of H2O2 solution containing x % of H2O2 by weight required x ml of KMnO4 for complete oxidation in acid medium. What is the normality of KMnO4 solution?


    2. Relevant equations

    H2O2 + KMnO4 => 2KOH + 2MnO2 + 2O2

    3. The attempt at a solution

    1 g of H2O2 solution contains x% of H2O2.

    Molar mass of H2O2 = 34 g, which means in 100 g of solution, 34g of H2O2 are present. In 1 g of solution, 0.34 grams of H2O2 are present. This reacts with 34 ml of KMnO4.

    I know that 17 g of H2O2 will react with 158.03 g of KMnO4, based on law of fixed proportions. The problem here is only the volume has been given and not the weight. The formula for Normality i.e. 1 mol / 1 L whereby 34 ml or 0.034 L is substituted in the denominator is not correct.

    I know I am doing something wrong, but can't figure out what it is. Any help is appreciated.

    The answer is 0.58 N.

    I am struggling with stoichiometry and I hope that I will be allowed to post thorny (from my point of view) problems in this thread.
     
    Last edited: Jul 5, 2011
  2. jcsd
  3. Jul 5, 2011 #2

    Borek

    User Avatar

    Staff: Mentor

    First of all - your reaction equation is wrong. Permanganate in acidic solution gets reduced to Mn2+.

    No, 1g of the solution contains 0.x g of H2O2

    Please start a new thread for each problem.
     
  4. Jul 5, 2011 #3
    Can you give the equation?

    I am beginning to understand. Guess my whole understanding was off. Since it says that the solution contains x % of hydrogen peroxide, I can't assume it as 34% by weight. Is this correct?

    The solution is to take the weight of hydrogen peroxide as x/100 in 1 gram. Since the equivalent weight of H2O2 is 17, the number of equivalence in this solution would be x/1700.

    How do I proceed beyond this?
     
  5. Jul 6, 2011 #4

    Borek

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    Staff: Mentor

    Come on, you can. And if you don't know how to balance the equation - its time to learn it. Products are Mn2+, H2O and oxygen. Potassium is just a spectator, if you will balance net ionic equation it will not interfere. And don't forget solution is highly acidic, you will need a lot of acid on the left.

    2xYes.

    Express number of equivalents in x mL of the titrant solution as a function of x and concentration. You will have two equations in two unknowns.
     
  6. Jul 6, 2011 #5
    Equivalents in KMnO4 will be Nx/1000

    x/1700 = Nx/1000

    N = 10/17 or 0.58 N.

    Got it, thanks.
     
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