What is the mass in grams of 1L of helium gas as STP? (Find moles and then convert moles to grams; atomic mass =4.0grams/mol) What is the mass in grams of 1 L of air at STP? (Air is 80% N2 and 20% O2, so the apparent molecular weight of air is 0.8 x 28 + 0.2 x 32=28.8g/mol) Why do helium balloons float? please show work