A gas mixture with a total pressure of $745 \mathrm{mmHg}$ contains each of the following gases at the indicated partial pressures: $\mathrm{CO}_{2}$, $125 \mathrm{mmHg}$ : Ar, $214 \mathrm{mmHg}$; and $\mathrm{O}_{2}, 187 \mathrm{mmHg}$. The mixture
also contains helium gas. What is the partial pressure of the helium gas? What mass of helium gas is present in a $12.0$ - $\mathrm{L}$ sample of this mixture at $273 \mathrm{~K}$ ?