Chemistry Question: Helium gas is contained in a tank with a pressure of 12.4MPa:

Helium gas is contained in a tank with a pressure of 12.4MPa. If the temperature inside the tank is 32.8°C and the volume of the tank is 18.0L (Recall that the prefix, capital M refers to ‘mega’)

Determine the number of moles of helium in the tank.
Determine the mass of helium in the tank.
What is the density of helium in the tank?

Thanks!

assuming that this is behaving like an ideal gas, use the formula

PV = nRT

you are given Pressure, Volume, and temperature (R is constant)

now just convert everything to famililar units. i like to use R = 0.08206, but that calls for pressure as atm, liters, and kelvin

12.4 MPa = 12.4*10^6 Pa = 122.4 atm

32.8 C = 305.95 K

so... using PV = nRT, move around the units to get n (number of moles)

n = PV/RT

n = (122.4)(18.0)/(0.08206)(305.95) = 87.76 mol He


now, using the number of moles and the molar mass, find the mass of the helium. it's approximately 351 g

finally, get density by diving mass over volume.

351 g / 18.0 L = 19.5 g/L