Calculate difference, δh-δe=δ(pv) for the combustion reaction of 1 mole of heptane. (assume standard state conditions and 298 k for all reactants and products.) the ideal gas law must be used. the incremental change in volume due to liquids is neglected (cf the volume of 1 mole of h2o(l) is 18 ml, whereas the volume of 1 mol of a gas at 298 k is about 24,450 ml. be careful to account for the total change in number of moles of gas molecules (be sure the signs and units are correct too). note that certain alkanes are gases in the standard state.