A gas mixture consists of N2,O2 and Ne, where the mole fraction of N2 is 0.55 and the mole fraction of Ne is 0.25. If the mixture is at STP in a 5.0 L container, how many molecules of O2 are present?

First, what is the mole fraction that is O2? =X
0.55 + 0.25 + Z = 1

Second, at STP, the volume of 1 Mole is 22.4 L
5 /22.4 = Y

Third, 1 mole is 6.02E23 molecules
so 6.02E23 * Y = # of molecules in 5 L container

& 6.02E23 * Y * Z = your answer