Can the change in internal energy of an ideal gas be non-zero during an isothermal process?

During the isothermal expansion,a confined ideal gas does $(-150) \ J$ of work against its surroundings. This means that

The temperature of an ideal gas is kept constant as it expands. The gas does external work. During this process,the internal energy of the gas

In which of the processes does the internal energy of the system remain constant?

In an isothermal process,which statement is wrong?

Two identical vessels $A$ and $B$ with frictionless pistons contain the same ideal gas at the same temperature and the same volume $V$. The masses of gas in $A$ and $B$ are $m_A$ and $m_B$, respectively. The gases are allowed to expand isothermally to the same final volume $2V$. The changes in pressures of the gas in $A$ and $B$ are found to be $\Delta P$ and $1.5 \Delta P$, respectively. Then