Question

Two Flasks containing the same inert gas are at the same temperature and pressure of 800 mmHg. One flask has volume of 1.0 L and the other, a volume of 2.0 L. Enough volatile liquid is injected into each of the flasks to allow phase equilibrium to be established. No leakage occurs. If the pressure in the 1.0 L flask is 900 mm Hg, what is the pressure in the 2.0 L flask?

Jason--
When Bob Pursley answered your question this morning (it is much clearer now when you reposted--thanks) he told you the pressure would be the same. That means that the pressure in the 2L flask will be 900 mm Hg.
You can use PV=nRT for the 1L flask and calculate the mols of inert gas (assume any T but keep it constant) at 800 mm Hg pressure. Then do the same for 900 mm Hg pressure. Subtract the two to determine the number of mols of the new gas from the volatile liquid. Then calculate n for the mols of inert gas in the 2L flask, add twice the mols of volatile liquid gaseous molecules, and calculate the pressure. It will be 900 mm Hg. I hope this helps.