# What are the partial pressures exerted by each component exerted by a 4:1 mixture of oxygen and argon with respect to moles if the total pressure is 1*atm?

$S T P$ specifies a temperature of $273.15 K$, and a pressure of $1 \cdot a t m$. (Or near enough to it; you should check your definitions.)
$\text{Dalton's Law of partial pressures}$ states that in a gaseous mixture, the partial pressure exerted by any component is the same the pressure it would exert if it alone occupied the container. The total pressure is thus additive, the sum of the individual partial pressures.
So if you have $4 : 1$ mix of dioxygen and helium gases, and the total pressure is $1$ $a t m$, then clearly ${P}_{\text{dioxygen}} = 0.80 \cdot a t m$, and ${P}_{\text{helium}} = 0.20 \cdot a t m$. Anyway, you should check on your definition of $\text{STP}$, they are different across different curricula.