The change in internal energy of the gas is [tex]\Delta U = 2.0 \cdot 10^4 J[/tex].
In fact, the 1st law of thermodynamics states that the change in internal energy of a system is equal to the amount of heat given to the system (Q) plus the work done on the system (W):
[tex]\Delta U = Q+W[/tex]
In this example, no work is done on the bottle so W=0, while the heat given to the system is [tex]Q=2.0 \cdot 10^4 J[/tex], so the change in internal energy of the gas is
[tex]\Delta U = Q = 2.0 \cdot 10^4 J[/tex]
