Question: Consider two cylinders of gas identical in all respects except that one contains O2 and the other He. Both hold the same volume of gas at STP and are closed by a movable piston at one end. Both gases are now compressed adiabatically to one-third their original volume. Which gas will show the greater pressure increase?
The "answer" is apparently that He shows the greater pressure increase since it is lighter. Gases with lower molecular masses will travel at a faster rate. Therefore, their pressure will increase at a faster rate.
Using the ideal gas law PV = nRT, I assumed that V, n, R, and T were all identical since the question mentions "identical in ALL RESPECTS". So just through this, I figured that the pressure has to be identical. Did I do something wrong?
Also, if the "answer" is actually correct, which equation would prove it?