Will chemo help pancoast tumor shoulder arm pain?

Pancoast. It depends. If pressure is lessened on the surrounding tissues, nerves, vessels, then arm pain could lessen.
The optimal . Treatment for pancoast tumor with symptoms is chemotherapy plus radiotherapy followed by surgery (in many if not most cases). Either modality alone is inadequate and less likely to control pain. Some tumors cannot be resected, and chemoradiotherapy is the fall back treatment. Survival is quite good, better than most lung cancers, .
Yes. This is bad pain and today's chemo will probably give you a few months of relative comfort. You may also be offered radiation. Discuss your options and remember you have a right to several physicians' ideas. Good luck.