A troy pound is equal to 12 troy ounces and to 5,760 grains, that is exactly 373.241 7216 grams. Troy weights were used in England by jewellers. Apothecaries also used the troy pound and ounce, but added the drachms and scruples unit in the Apothecaries' system of weights.
Depends on the API gravity of the gasoline, which varies by grade and refinery. Usually, regular unleaded gasoline has a gravity of around 58 and a weight per gallon of 6.216 pounds per gallon. Premium gasoline may have a gravity of 54, or 6.350 pounds per gallon.
So any gas with a MW less than 29 will be lighter than air. That, by the way, includes nitrogen! Methane has a MW of 16; helium is 4; hydrogen is 2. I can't think of any other compound or element that's a gas at STP that has a MW less than 29. Afterthought: OK, neon has a molecular weight of 20, which is lighter than air. Ammonia (gas at room temp) has a MW of 17.