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Homework Help: Percent by Mass Chemistry Question

  1. Sep 4, 2010 #1
    1. The problem statement, all variables and given/known data

    A thoroughly dried 1.283 sample of Na2SO4 is exposed to the atmosphere and found to gain 0.395 in mass. What is the percent, by mass, of Na2SO4 * 10H20 in the resulting mixture of anhydrous and the decahydrate?

    2. Relevant equations
    1 mol Na2SO4 = 142.0428 g
    1 mol H20 = 15.994 g

    3. The attempt at a solution

    Don't really know where to start. I found the moles of Na2SO4 in 1.283 g sample (.009032) and the moles H20 in sample once it had gained weight (1.678 sample, .118133 mol H20)
     
  2. jcsd
  3. Sep 5, 2010 #2

    Borek

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    Staff: Mentor

    If you know number of moles of water, can you calculate number of moles of hydrate that was produced? Number of moles of anhydrous sulfate left?
     
    Last edited by a moderator: Aug 13, 2013
  4. Sep 5, 2010 #3
    I found the moles hydrate in the .11813 mol H2O by dividing it by 10 mol h2o per mol hydrate. Then I divided that by the 1.678 g hydrate mixture (1.283 +.395 g gained) to find the percentage mass. But that was not right either
     
  5. Sep 5, 2010 #4

    Borek

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    Staff: Mentor

    Judging from what you wrote you divided number of moles by mass. You need to divide mass by mass.
     
    Last edited by a moderator: Aug 13, 2013
  6. Sep 5, 2010 #5
    I saw the mistake after I posted it and converted the moles hydrate to g, but the resulting mass was greater that 1.678 (giving a percentage greater than 100)

    Basically what I have to do is find
    g hydrate (Na2SO4*10H2O) / 1.678 g mixture
    and multiply that by 100 to get ther percentage by mass, right?
    From that, I tried again working backwards to find moles H20 like you said. I converted 1.678 g hydrate to moles H20 and got .52105 mol H20. Now I'm not sure where to go from there; I tried converting that to grams hydrate but ended up back to 1.678 g of course.
     
  7. Sep 5, 2010 #6

    Borek

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    Probably some math error.

    Please list:

    number of moles of water
    number of moles of hydrate
    mass of the hydrate

    as you have calculated.
     
    Last edited by a moderator: Aug 13, 2013
  8. Sep 5, 2010 #7
    .052105 mol water
    .00521 mol hydrate
    1.89059 g hydrate
     
  9. Sep 5, 2010 #8

    Borek

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    0.052105?
     
  10. Sep 5, 2010 #9
    Do I convert .395 g to moles water to find the moles water (since all of the mass gained is water)?
     
  11. Sep 5, 2010 #10

    Borek

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    Staff: Mentor

    What else can you use?
     
    Last edited by a moderator: Aug 13, 2013
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