How many molecules of 02 are in 3.21 grams of 02?

Question

I know oxygen is diatomic which means you have to double its atomic weight since there are two molecules of oxygen, but i m not sure how to set up the problem. HELP PLEASE! 10 points for best answer :]

Answer 1

Ok, so from the information given in the question you know that Oxygen's given mass= 3.21g

molecules?

Since you want to know how many molecules of O2 are in 3.21g of O2 you have to use Avagadro's Number which is 6.02 X 10^23

As you said, first you have to get Oxygens actual mass by multiplying its atomic mass (its 16) by 2, which equals 32g O2

Now you have:

3.21g O2 (given mass)

32g O2 (actual mass) and

6.02 X ^23

Since its from grams to molecules, the way you set it up is like this:

3.21g O2 *6.02 X ^23 / 32g O2 = 6.03 X ^22

meaning that 6.03 X ^22 molecules are in 3.21g of O2

Answer 2

Alright, you know that O2 has a molar mass of 32.00 g/mol.  You've got 3.21 g, so all you have to do is divide (g/g/mol = mol) to get how many mol you've got.

3.21 / 32.00 ~ .10 mol  (You'll have to do the actual math; I don't have a calculator)

Multiply the number of mol you have in the 3.21 g O2 by Avagadro's Number (6.02 x 10^23) and you'll know how many molecules you've got in 3.21 g O2.

I think you'll get something like 6.02 x 10^22.