To calculate the entropy change for a monatomic ideal gas during an isothermal expansion, we can use the formula:S = n * R * ln V2/V1 where:S is the entropy change,n is the number of moles of the gas,R is the ideal gas constant 8.314 J/molK ,V1 is the initial volume,V2 is the final volume, andT is the temperature in Kelvin .Since the process is isothermal, the temperature T remains constant throughout the expansion. Note that this formula is derived from the equation for the entropy change of an ideal gas during a reversible isothermal process.