How do I find the density of helium in a 2 L tank at one ATM and 23°C

Answer 1

Since density is defined as mass/volume, and you know the volume is 2 L, you now must find the mass. From the Ideal Gas Law, you can calculate moles (n) and hence you can find the mass.

PV = nRT

Solving for moles (n):

n = PV/RT = (1 atm)(2 L)/(0.0821 Latm/Kmol)(296K) note temperature is in K

n = 0.082 moles

mass of He = 0.082 moles x 4 g/mol = 0.3 g (to 1 significant figure)

Density = 0.3 g/2 L = 0.15 g/L = 0.2 g/L (to 1 significant figure)