Propane (C3H8) and butane (C4H10) have molecular weights of 44 and 58 g/mol, respectively, so both will be heavier than air.Natural gas is a mixture, not a compound, and its composition varies considerably depending on its source. The natural gas delivered to consumers in the U. S. is about 95% methane (CH4, molecular weight 16 g/mol) with the remainder being mostly ethane, propane, and carbon dioxide. A balloon filled with natural gas rises in air, although it doesn't have the lift a helium balloon has.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/gases/faq/print-heavier-or-lighter-than-air.shtml