A balloon is filled with helium gas at a pressure of 2 atm and a volume of 10 liters. If the gas is then compressed to a volume of 5 liters, what is the new pressure of the gas, assuming the temperature remains constant? Additionally, if the gas was initially at 300 K, what would be the final temperature in Kelvin if the balloon was allowed to expand back to 10 liters at a constant pressure of 2 atm? Let's think step by step and output the final answer within \boxed{}.
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