So knowing that C=12.011 and O=15.999 (assuming that they are averages since no isotope was disclosed in the problem) then one molecule of CO2 is equal to 12.011 x 2(15.999)=44.009, so knowing that one CO2 molecule is equal or approximately 44.009 amu (atomic mass unit) then we can use this simple formula – 44.009x= (the mass of one CO molecule) x 3.5 x 10^6

or

44.009x=98035000 and divide 98035000 by 44.009 to get a raw answer of 2227612.5337999 or rounded to the significant digits = about 2,227,612 molecules of CO2.

1 molecule of CO weighs – 12+16=28 amu

3.5 million molecules will weigh – (28×3.5) x 1 million = 98 million amu

weight of 1 molecule of CO2 = 12+16×2 =44

no of molecules of CO2 in 98 million amu = (98x 1 million)/ 44 = 2.227 million molecules.

hope it helps