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Homework Help: Empircal composition question

  1. May 12, 2009 #1
    1. The problem statement, all variables and given/known data
    I am running an experiment in which I have a compound of SiO2 Na2 K2O PbO I am trying to figure out the percent composition of oxygen given that the percent composition of the compound is:
    46% SiO2
    5% Na2
    4% K2O
    and 45% PbO

    2. Relevant equations

    46% SiO2
    5% Na2
    4% K2O
    and 45% PbO

    3. The attempt at a solution
    So I figured given the previous percentages, the compound is made of
    [tex] .46 SiO_2 * \frac{2 mol O }{1 mol SiO_2} + .04 K_2O * \frac{1 mol O}{1 mol K_2O} + .45 PbO * \frac{1 mol O}{1 mol PbO} = .46*2+.04+.45 = 1.41 [/tex]

    but this gives me 141% oxygen composition, what am I doing wrong here? Its been a while since I took general chem (about 3 years ago) and my stoichiometry is not at its best right now.
    Last edited: May 12, 2009
  2. jcsd
  3. May 12, 2009 #2
    would it be more correct to say that the percent composition of oxygen is:

    [tex] 0.46* \frac{2}{3}O + .04* \frac{1}{2}O+ 0.45* \frac{1}{2}O [/tex]
    = .5516 = 55.2%?
  4. May 12, 2009 #3
    or would it be easier if I do the calculation by breaking it down into atomic masses, add the total of oxygen mass divided by the total mass of the compound?
  5. May 12, 2009 #4
    thanks fellas, once again, couldn't have done it without yall.
  6. May 13, 2009 #5


    User Avatar

    Staff: Mentor

    You are adding apples and oranges. Each of these oxides contains different amount of oxygen (mass of oxygen per mass of the oxide), you can't add them this way. As you have already (and correctly) noticed, that gives idiotic results (percentage above 100%).

    Your last approach is correct. Think this way: SiO2 is 46% of the substance. Assume you start with 100 g of the substance. How many grams of SiO2? How many grams of O in these grams of SiO2? (That's where the molar masses come into play).
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