The first law of thermodynamics states that:
[tex]\Delta U = Q-W[/tex]
where
[tex]\Delta U [/tex] is the variation of internal energy of the gas
Q is the heat absorbed by the gas
W is the work done by the gas
The container has a fixed volume, so there is no work done on/by the gas (because the work is proportional to the variation of volume: [tex]W=p \Delta V[/tex]). Therefore, W=0, and the equation becomes
[tex]\Delta U = Q[/tex]
and the variation of internal energy of the gas is simply equal to the heat added to it:
[tex]\Delta U = Q = +200 J[/tex]
