3 moles of an ideal gas initially at a pressure of 1 atm and a temprature of 100°C are expanded to twice the initial volume and then heated to 200°C at the new volume.What is the final pressure in atm?

Answer 1

(P1V1/T1) = (P2V2/T2)
You don't need to use moles of gas if you use this formula. Notice that you have no initial volume and no actual final volume (just twice the initial). I would pick a convenient number (any number) for V1 and multiply that by 2 for V2.