A mixture of gases containing 0.2 moles of O2, 0.3 moles of N2, and 0.5 moles of H2 is collected at STP. If 20% of the H2 escapes due to a leak, what is the final volume occupied by the remaining gas mixture?

A flask containing a mixture of He and CH4 is maintained at STP. If the partial pressure of He is 0.4 atm, what is the mole fraction of CH4?