Does the internal energy of an ideal gas change in an isothermal process?

What is the relationship between pressure $P$ and volume $V$ for an ideal gas undergoing an isothermal process?

$Assertion :$ The isothermal curves intersect each other at a certain point.
$Reason :$ The isothermal change takes place slowly,so,the isothermal curves have very little slope.

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

What is the value of the specific heat of a gas during an isothermal process?

During an isothermal expansion,a confined ideal gas does $-150 \, J$ of work against its surroundings. This implies that