If three moles of an ideal gas at 300K expand isothermally from 30...

Given,

 ${V}_{1} = 30{dm}^{3}{V}_{2} = 45{dm}^{3}P = 80kPa$

Using, first law of thermodynamics,
$\Delta U = Q + W$

$Q$ is the heat given or lost

$\Delta U$ is the change in internal energy

$W$ is the work done

$\Delta U = 0$ : Process is isothermal

$Q = - W$

$W = - {P}_{ext}\Delta V:$ Irreversible

$W = - {P}_{ext} ( {V}_{2} - {V}_{1} )$

$= - 80 \times {10}^{3} ( 45 - 30 ) \times {10}^{- 3}$

$= - 1200J$

Hence, the amount of heat transferred is $1200J$