# Kaplan blue page 671, #49 Gen CHEM

#### joonkimdds

##### Senior Member
10+ Year Member
7+ Year Member
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?

#### dental#1

##### Fla DDS
10+ Year Member
joonkimdds said:
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?
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.

#### issa

##### Senior Member
10+ Year Member
5+ Year Member
joonkimdds said:
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?

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.

#### dental#1

##### Fla DDS
10+ Year Member
issa said:
200g sample * 1mole sample/total sample weight * 9 mole O/ 1 mole sample * 1 mole O/ 16g O = answer

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

#### joonkimdds

##### Senior Member
10+ Year Member
7+ Year Member
issa said:
200g sample * 1mole sample/total sample weight * 9 mole O/ 1 mole sample * 1 mole O/ 16g O = answer

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.
Isn't the oxygen % of oxygen same in both 200g and 2000g of water because H2O always have H:O = 2:1 ratio?

#### issa

##### Senior Member
10+ Year Member
5+ Year Member
joonkimdds said:
Isn't the oxygen % of oxygen same in both 200g and 2000g of water because H2O always have H:O = 2:1 ratio?
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.