Compute Gas Density
The molar mass of helium is 4.003 g/mol. What is its density in g/L at STP?
density = grams/liter

1. Assume a volume of 1.00 L
2. Compute the mass of 1.00 L at STP
moles at STP = n = (PV)/(RT) = (1 atm)(1 L)/(0.0821 L atm/mol K)(273 K) = 0.0446 mol
mass = grams = (0.0446 mol)(4.003 g/mol) = 0.179 g
density = 0.179 g/1.00 L = 0.179 g/L


Here's a short cut. 22.4 L at STP is one mole which must weigh 4.003 grams, right? Then density = 4.003 g/22.4 L = 0.179 g/L
What is the density of air at STP?
Air is about 20% O2 (32.0 g/mol) and 80% N2 (28.0 g/mol) and has an average molar mass of (0.2 x 32) + (0.8 x 28) = 28.8 g/mol. The density of air at STP is (28.8 g/mol)/(22.4 L/mol) = 1.29 g/L
Now explain why a helium ballon floats in air.