# Mass Of Oxygen

1. Nov 20, 2015

### Sace Ver

1. The problem statement, all variables and given/known data
What would be the mass of oxygen in 2.5 x 102g of CaCO3?

Given:
mCaCO3=250g
M=100.09g/mol
n=2.498mol

2. Relevant equations
?

3. The attempt at a solution

mass of O=250g/3 atoms of O
mass of O=83g

Is that correct?
Bc if it is can someone please explain why it's correct.

2. Nov 20, 2015

### SteamKing

Staff Emeritus
Remember, 250 g of CaCO3 contains more than just Oxygen. What about the Calcium and the Carbon in the substance? What happened to them?

3. Nov 20, 2015

### Sace Ver

So oxygen wouldn't be 83g?

4. Nov 20, 2015

### SteamKing

Staff Emeritus
Honestly, what do you think?

You can't divide the mass of a substance by the number of atoms of one constituent to obtain the mass of that constituent if other kinds of atoms are present. That should be understood, whether you're taking chemistry or not.

5. Nov 20, 2015

### Sace Ver

Not to put the blame on others someone showed me how to do it that way which made me confused.

So I'm assuming the final answer is 119.9g of Oxygen.

6. Nov 20, 2015

### SteamKing

Staff Emeritus
And how did you arrive at that figure?

7. Nov 20, 2015

### Sace Ver

Given:
mCaCO3=250g
MCaCO3=100.09g/mol
nCaCO3=2.498mol

Required:
mO=?

Solution (I tried two methods):

Method #1:

%decimal= 16(3)/100.09g/mol= 0.4796

(0.4796)(250g of CaCO3)= 119.9g of O

Method #2:

250gCaCO3/100.09g/mol=2.498mol

2.498mol x 3 molO = 7.494molO

7.494molO x 16gO = 119.9 g of o

8. Nov 20, 2015

### SteamKing

Staff Emeritus
Method 1:
0.4796 is the fraction of the mass of CaCO3 which is composed of Oxygen. This fraction remains the same regardless of the how much CaCO3 you are given.

0.4796 is also called the ratio of the mass of Oxygen to the mass of CaCO3.

Now, do you see why your original method of calculation in the OP was faulty?

Method 2:
There are 2.498 mol of CaCO3 in 250 g of the substance. There are 3 atoms (not moles) in each molecule of CaCO3 .

That's why there are 2.498 mol × 3 atoms of O / molecule = 7.494 moles of Oxygen in 250 g of CaCO3

9. Nov 21, 2015

### Sace Ver

I now see why the original calculation was incorrect.