Question

At STP a container has 1 mole of Ar, 2 mole of CO2, 3 moles of O2 and 4 moles of N2. Without changing the total pressure if one mole of O2 is removed, the partial pressure of O2 is :

Answer 1

at the start there are 10 mol of gas 
and the P_O2 = 3/10 atm 
after the removal there are 9 mol of gas 
and P_O2 = 2/9 atm 
then 
2/9 / 3/10 = 20/27 = 0.74 
or 74% of the initial value

Answer 2

The partial pressure of O2 is 0.74