Carbon's atomic mass is 12.011. Oxygen's is 15.9994, therefore the atomic mass of a single molecule of CO
2 is 12.011 + (2 x 15.9994) = 43.999.
Compare this to a molecule of H
2O; hydrogen's atomic mass is 1.00794, therefore water has an atomic mass of (2 x 1.00794) + 15.994 = 18.00988.
Ever notice that a chunk of dry ice sinks to the bottom of a bowl of water? 'Nuff said, however, here's an interesting read:
http://vulcan.wr.usgs.gov/Glossary/Lakes/description_volcanic_lakes_gas_release.html 