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DAT Test Prep Material Discussion Free Dental Admissions Guide Article: Requirements Tapatalk

Kaplan blue page 671, #49 Gen CHEM

Discussion in 'DAT Discussions' started by joonkimdds, Jul 27, 2006.

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  1. joonkimdds

    joonkimdds Senior Member
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    what is the percentage of oxygen by mass in a 200gram sample of CuSo4 * 5H2O?

    I just added all the masses, got the mass of oxygens.

    mass of oxygen / total mass * 100 % = percentage of oxygen in CuSo4 * 5H2O.

    What I don't understand is why does it say 200 gram sample?

    mass of oxygen = 144
    total mass = 250

    is it because no matter what the total mass is, the percentage of oxygen doesn't change becuz the mass of oxygen will also inc or dec at the same rate as the total mass?
     
    #1 joonkimdds, Jul 27, 2006
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  2. dental#1

    dental#1 Fla DDS
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    I think your thinking is on the right path. But what was your answer?72% or 57.6%. I am thinking.... maybe you should do this( I am not sure i am looking it up!)

    ([given]gof compound)(mole/gcompound)= moles of compound

    (molcomp)(moleO/molcomp)(gO/1mol)=gO

    massO/Total*100%=?

    g/250g*100= % or g/200g*100%= %. Let me know. this is either really easy or tricky
    Kaplan doesn't explain this well :mad: I originally would think to do it just massofO2/total mass but the 200g is throwing me.
     
    #2 dental#1, Jul 27, 2006
  3. issa

    issa Senior Member
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    200g sample * 1mole sample/total sample weight * 9 mole O/ 1 mole sample * 1 mole O/ 16g O = g oxygen/total sample weight * 100 = % O

    it says 200g sample because it is using specific amount of sample. think about what if you use 200g water instead of 2000g water what will happen? in the 2000g water the oxygen % will will a lot higher than the Oxygen level in the 200g sample.
     
    #3 issa, Jul 27, 2006
  4. dental#1

    dental#1 Fla DDS
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    Good I am glad to know I was on the right track!!!!!!!!!!!!!!!!
     
    #4 dental#1, Jul 27, 2006
  5. joonkimdds

    joonkimdds Senior Member
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    Isn't the oxygen % of oxygen same in both 200g and 2000g of water because H2O always have H:O = 2:1 ratio?
     
    #5 joonkimdds, Jul 27, 2006
  6. issa

    issa Senior Member
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    yes,
    i just looked in the kaplan blue book and it says that we don't have to worry about the 200g or any sample size, so the size of the sample doesn't affect the oxygen compoistion percentage.
     
    #6 issa, Jul 27, 2006
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