1. The problem statement, all variables and given/known data Heat Q flows into a monatomic ideal gas, and the volume increases while the pressure is kept constant. What fraction of the heat energy is used to do the expansion work of the gas? 2. Relevant equations ΔU = Q - W Q = nC_pΔT W = PΔV C_p = C_v + R => C_p = 3R/2 + R => C_p = 5R/2 3. The attempt at a solution So we sub in the Q and W in the internal energy formula and we get: ΔU = nC_pΔT - PΔV ΔU = (5/2)(nRΔT) - PΔV ΔU = (5/2)(PΔV) - PΔV ΔU = (3/2)(PΔV) Therefore we can take ΔU/Q to find the fraction: (3/2)(PΔV)/(5/2)(PΔV) Which gives me 3/5. However the answer is 2/5. What am I doing wrong? Personally, I think the answer key is wrong. Thanks.