ANSWERS: 2

If you put in another mole the pressure would increase to 2 atm at 273K. You could do the same by reducing T to 136.5K and 1 atm. There are an infinite number of combinations of pressure and T change that could accomplish the same.

PV=nRT should cover your needs here, to be honest. You have a known volume, temperature, etc. P1V1=P2V2. You know p1, v1, and v2, so just rearrange that to get to P2. Next, take PV=nRT and rearrange to find T.
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