If your arm hurts, you should pyut down the piece and play it later. That particular rhapsody is very demanding and needs a technique that lets you relax, especially your wrist, even in great tempi. Which means that even in the octave leaps, you should not strain anything but what is important. And everytime you lay your arm flat, you should relax even for just a millisecond. That takes time to learn and you should probably wait with the piece and pick it back up later