Change in Internal Energy during Isothermal Expansion

Consider a closed system containing an ideal gas that is undergoing an isothermal expansion. The gas expands from an initial volume of 2.00 L to a final volume of 5.00 L at a constant temperature of 300 K. The process is done against an external pressure of 1.00 atm. Use the ideal gas constant R = 0.0821 L·atm/(K·mol).

What is the change in internal energy (ΔU) of the gas during this isothermal expansion?