Unfortunately, "it depends" For many tax liabilities you have responsibility directly to the government agency as a "responsible person". In some cases, even the business liability shield won't protect you. You may have indemnity rights against your former partner under the purchase agreement. Of course, if he has no money, those rights are of little use, and you may be out of pocket if the goverment is successful in the claim against you. The answer to your question really requires careful review of the source and nature of the liability, the terms of the Purchase Agreement, and the related circumstances.
You really should engage and attorney in this. If the liability is not sufficent to justify hiring professional help, you need to plead your case to the goverment agency, paying very careful attention to the procedural requirements to oppose their claim.
Did you officially separate from the "partnership"? You should have filed a public notice that effective Month DD, YYYY you are no longer a partner of or responsible for its debts. That being said, you are in all liklihood responsible for debts accrued prior to your separation. You need to deal with the IRS to work out exactly what your responsibilities are. It is often necessary to hire and attorney and/or CPA to deal with the IRS on your behalf (depending on how much is at stake).
I warn every client and potential client, "NEVER, NEVER, NEVER speak with a government official without counsel present or a complete grant of immunity in effect."