1. The problem statement, all variables and given/known data I want to calculate the energy of an inlet gas stream (I will add heat later but need to know the rate at which energy is coming into the system [J/s]). The fluid is air and I can decompose it into molar flow rates of the constituent gases. I now need to use lookups to find the energy of each constituent gas. I believe what I'm after here is a state variable because its based on the ambient condition and not any process that happened beforehand. 2. Relevant equations I can think of 2 equations here: Q = m_dot * Cp * T Q = m_dot * H (where H = enthalpy) 3. The attempt at a solution I have found some lookup data for values of H and Cp of oxygen and nitrogen. However the 2 equations give different plots; when I plot T vs H and T vs (Cp * T) the plots are similar but significantly different. I'm not sure why they are different or which one I should use. Also, is what I'm trying to calculate defined as the internal energy of the gas? Thanks for your help.