In a litre flask at a certain temperature, there are 2 gm of $H_{2}$ and 8 gm of $O_{2}$. The mole fraction of $O_{2}$ in the given mixture is:

First, convert the masses to moles by dividing by the molar masses, yielding 1 mole of hydrogen (2 g divided by 2 g/mol) and 0.25 moles of oxygen (8 g divided by 32 g/mol). The mole fraction of oxygen is its moles divided by the total moles in the mixture. Calculate this as 0.25 divided by 1.25, which equals 0.2.

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