How to determine an unknown gas in a tube with a massless piston at equilibrium conditions?


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> There is a massless piston that can move freely in a horizontal tube that is closed at both ends. To the left, there is a sample of helium gas at $300~\mathrm{K}$ and $4~\mathrm{atm}$. To the right, there is a sample of an unknown gas $\ce{X}$ at $500~\mathrm{K}$ and $2~\mathrm{atm}$. At equilibrium, the piston is located at 1/9 the length of the entire piston from the right side. The molecules of $\ce{X}$ are homonuclear. What is most likely the identity of $\ce{X}$?
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(From Chemistry Olympiad)


I'm confused by the equilibrium part. I tried $PV = n\mathcal{R}T$ to find the ratio of moles. But without molar mass, how do you solve for it? How would you solve it?


