1. The problem statement, all variables and given/known data 500 Cal are added to a gas inside a cylinder with a piston (containing one mole) by an external heating device. The volume of the gas doubles without any change in it's temperature of 300K. How much work is done on the piston? 2. Relevant equations 3. The attempt at a solution W = -nRT ln(2Vi/Vi) where ln is the natural logarithm W = -nRT ln(2) W = -(1)(1.987)(300) ln(2) = -413.2 Cal R = 1.987 as the problem is in Calories not Joules. As it's an isothermal expansion I thought this would be correct but the answer given in the text book is as follows: W = -500 - (1)(R)(300) ln2 = -500 - (1)(1.987)(300) ln2 = -913.185 Cal If 500 Cal is added to the gas and the gas expands isothermally why are there 2 terms in the solution? Basically I don't understand why we need to include the -500 Cal here.