How many oxygen atoms in 3.161*10^21 molecules of CO2?

You need to know one piece of information for these problems. That is as follows:

There are 6.02 x 10^23 atoms in a mole of atoms or 6.02 x 10^23 molecules in a mole of molecules (actually there are 6.02 x 10^23 in a mole of anything).

So in 3.161 x 10^21 molecules of CO2 there are 3.161 molecules x (1 mole CO2/6.02 x 10^23 molecules CO2) = ? moles CO2. Then multiply that by 2 to find the moles of O in CO2.

The others are done th same way.