Question

Calculate the partial pressure of 3.25 moles of oxygen that have been mixed (no reaction)
with 1.25 moles of nitrogen in a 25.0 L container of 298 K.

Answer 1

Use PV = nRT and substitute 3.25 for n if you want pO2.
Use PV = nRT and substitute 1.25 for n if you want pN2.
Use PV = nRT and substitute (3.25+1.25) if you want the total pressure in the container. (or add pO2 + pN2 = Ptotal).