At 298K, a flask 'A' of unknown volume (V) contains oxygen at 5 atm. Another flask 'B' of volume 2L contains helium at 3 atm. Two flasks are connected together by a small tube of zero volume. After the two gases are completely mixed, if the resulting mixture is found to have the mole fraction of oxygen as 0.2, the volume of flask 'A' (in L) is (Assume oxygen and helium as ideal gases)
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For problems involving mixing ideal gases at constant temperature, you can use the fact that the number of moles, $n$, is directly proportional to the product $PV$. Use these proportional quantities ($n \propto PV$) to calculate mole fractions.