My mother is on Medicare when she passes away do i pay that Medicare bill back.?

You are not personally responsible. However, if your mother owns any property in her name the government can record a lien against her estate. The lien must be paid before any property can be distributed.

For future reference:

If it happens that a Medical Assistance lien is filed against your mother's estate a skilled attorney can usually negotiate a settlement for a lessor amount in return for a cash offer.