A gas mixture contains twice as many moles of O2 as N2. That means the mixture contain moles as 2 parts of O2 and 1 part of N2. Now we calculate the number of mole in the mixture:
The partial pressure of 0.200 mol of Ar = (1.10 - 0.800) atm = 0.30 atm.
Now the 0.800 atm contains 0.800 x(2/3) moles in the mixture which is 0.533 mol gas mixture of O2 and N2.
So two parts of gas mixture will be the moles of O2. So the answer is 0.356 mole of O2.
