1. The problem statement, all variables and given/known data I am to show that ΔS=Q/T for the isothermal expansion of a monoatomic ideal gas, when the expansion is so slow that the gas is always in equilibrium. 2. Relevant equations 1. law: ΔU=Q+W (We mustn't use dQ and dW - our teacher hates that :( ). Ideal gas law: PV=NkT We need the equation: ΔS=Nk*ln(V_final/V_initial) And that quasistatic expansion work is W=-PΔV 3. The attempt at a solution -I think I am to start with: ΔU=Q+W⇔Q=ΔU-W, where ΔU=0 since its isothermal. -I know that it is quasistatic expansion work, so W = -PΔV, so Q = -(-PΔV) = PΔV I think I want to get something from the ideal gas law in here: P=(NkT)/V, so Q=(NkTΔV)/V But then I kind of get stuck there... Hope someone can help. I thinks it is really easy, but I kind wrap my head around it.