Answer:
45.3 KPa 
Step-by-step explanation:
From the question given above, the following data were obtained:
Total pressure (Pₜ) = 101.3 KPa
Pressure of Hydrogen (Pₕ) = 56 KPa
Pressure of Oxygen (Pₒ) =? 
The pressure of oxygen in the mixture can be obtained as follow:
Pₜ = Pₕ + Pₒ
101.3 = 56 + Pₒ 
Collect like terms 
101.3 – 56 = Pₒ 
Pₒ = 45.3 KPa 
Thus, the pressure of oxygen in the mixture is 45.3 KPa