Kaplan blue page 671, #49 Gen CHEM

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

1. 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?

3. 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 I originally would think to do it just massofO2/total mass but the 200g is throwing me.

4. 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.

5. dental#1 Fla DDS

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Good I am glad to know I was on the right track!!!!!!!!!!!!!!!!

6. 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?

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