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Thread: Calculating moles question

  1. #1 Calculating moles question 
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    H2O2(aq) --> H2O(l) + O2(g)

    so I balanced it...

    2H2O2(aq) --> 2H2O(l) + 1O2(g)

    then it askes " Calculate the number of moles of oxygen gas produced from the complete catalyzed decomposition of 5.20ml sample of a 3.5% solution of H2O2. The density of the 3.5% solution of H2O2 is 1.01 g/ml. "

    I find the mass in grams and get the moles and multiply by 3.5% and then use the molar ratios, but I am not getting the correct answers?

    any Ideas? Thanks

    -Chris


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  3. #2  
    Moderator Moderator AlexP's Avatar
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    regrettably I don't have an answer for you, sorry. What class is this in? I'll be doing something similar on Monday in AP Bio.


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  4. #3  
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    Chem 111
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  5. #4  
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    I think that the trick to that question is to realise that of the 5.20ml H2O2 only 3.5% (0.182ml) is actually hydrogen peroxide.

    From there it should just be simple stoich.
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  6. #5  
    Forum Freshman César's Avatar
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    I very much doubt that the 3.5% refers to volume. It would be most likely referred to weight, hence the necessity of the density.

    Best regards,

    César
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  7. #6 problem approach 
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    Assuming the percent H2O2 is mass/volume, then the equation should be able to be set up as

    5.20mL H2O2(aq) x 3.5g H2O2/100mL H2O2 x 1 mole H2O2/18 g H2O2 x

    1mole O2/2 moles H2O2 = moles O2 produced

    Does anyone see any problem with the setup?

    Cheers,
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    www.wellstonesolutions.com/forsale
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  8. #7  
    Forum Freshman César's Avatar
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    Does anyone see any problem with the setup?
    Yes.
    A) 3.5% is an adimensional number, i.e., has no units as it is the result of of a division between values with the same units.
    B) You make no use of the density.

    From my point of view it should be like follows:

    5.20mL · 1.01g/mL = 5.25g of solution
    5.25g · 0.035 = 0.184g of H2O2
    0.184g / 18g/molH2O2 = 0.010 molH2O2
    0.010 mol of H2O2 represents 0.005 mol of O2

    Best regards,

    César
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  9. #8 one correction 
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    The above answer is correct except for the 18g thats for H2O ... H2O2 is 34
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  10. #9  
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    Ooops!

    Thank you!
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