If the solvent mixture has a chart entalpy-concentratrion like a ethanol-water mixture, you should read the difference between the solution with the concentration final and initial, so you have the entalpy specific (btu/lbmol for example) then you will multiply that and the differences of volume tha you have. Seek the chart is important
The point I can add is that, if th table is molar, you must multiply the enthalpy difference is number of moles and if it is mass based the same thing is true about mass. If you couldn't find the table, then you can use one of the EOSs(equations of states) like Ping Rabinson. The chose amone different equations of state is an engineering art. In orderer ro keep away complicated mathmatical process of the EOS calculation, you can use ASPEN or HYSIS.