Silane, SiH4, reacts with O2 to give silicon dioxide and water:

A 5.20 L sample of SiH4 gas at 356 mmHg pressure and 25 oC is allowed to react with O2 gas. What volume of O2 gas , in liters, is required for complete reaction if the oxygen has a pressure of 425 mmHg at 25 oC?

I understand PV=nRT in this case but we are not given the mols in order to solve for volume.

You are given p, V & T for silane so you can solve for moles. You know p, T & n for oxygen and can solve for the volume. Tell us how you do.