In this question, we need to find out how many molecules we have in 2.15 moles of CO, and in order to do that, we will use the Avogadro's number, 6.02*10^23, which is the value that represents 1 mol in atoms or molecules
1 mol = 6.02*10^23 molecules
2.15 moles = x molecules
x = 6.02*10^23 * 2.15
x = 1.29*10^24 molecules of CO in 2.15 moles