A mixture contains 0.5 mol He, 0.09 mol CO2, 0.75 mol O2 and 1.5 mol At. Which gas has the highest partial pressure?

At.

Explanation:

The partial pressure of a gas in a mixture is equal to the mole fraction of the gas multipplied by the total pressure.

The mole fraction of a component in a mixture is equal to the number of moles of the component divided by the total number of moles in the mixture.

In the mixture given there are:

0.5 mol He, 0.09 mol CO₂, 0.75 mol O₂, and 1.5 mol At.

Hence, the gas with greatest number of moles, and, consequetly, greatest mole fraction and highest partial pressure is At.