Question

The atmosphere in a sealed diving bell contained oxygen and helium. If the gas mixture has 0.200 atm of oxygen and a total pressure of 3.00 atm, calculate the mass of helium in 10.0 L of the gas mixture at 40 degrees Celsius.

Answer 1

I assume the T of the mixture is 40 C.
If pO2 = 0.200 atm and Ptotal = 3.0 atm, then pHe must be 2.8 atm.
Use PV = nRT to solve for n = number of mols, then n = grams/atomic mass. You know n and atomic mass; solve for grams. Don't forget that T must be in kelvin.

Answer 2

1.56 g He