A gas mixture with a total pressure of 750 mmHg contains each of the following gases at the indicated partial pressures: CO2, 253 mmHg ; Ar , 133 mmHg; and 02, 166 mmHg . The mixture also contains helium gas. What is the partial pressure of the helium gas? What mass of helium gas is present in a 11.o L sample of the mixture at 568 K?