As you can infer from the formula, one Na2CO3 molecule contains 2 atoms of sodium, 1 atom of carbon, and 3 molecules of oxygen. You can use this as a conversion factor, therefore, to infer that for every mole ofNa2CO3you have, you will have 3 moles of oxygen.
However, you don't have 1 mol of Na2CO3, you have 0.5. However, this is easily remedied using a simple conversion factor:
However, we're not done yet! We have 1.5 moles of oxygen, not atoms. However, we do know that 1 mole=6.022⋅1023 atoms. Hence, we can use that as another conversion factor:
1.5 mol oxygen⋅6.022⋅1023 atoms of oxygen1 mol oxygen
This leaves 9.022⋅1023 atoms of oxygen.
If you need more help on converting between moles and atoms and mass, here's some useful resources:
) 