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 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. |