Concentration total amount of all "other" constituents in a mixture: If formula_6 is much smaller than formula_26, the mole ratio is almost identical to the mole fraction. The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios. The mass fraction formula_33 is the fraction of one substance with mass formula_2 to the mass of the total mixture formula_35, defined as: The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions. The mass ratio formula_37 is defined as the mass of a constituent formula_2 divided by
Internal energy density called "specific internal energy" which is internal energy per unit of mass (kilogram) of the system in question. The SI unit of specific internal energy is J/kg. If the specific internal energy is expressed relative to units of amount of substance (mol), then it is referred to as "molar internal energy" and the unit is J/mol. From the standpoint of statistical mechanics, the internal energy is equal to the ensemble average of the sum of the microscopic kinetic and potential energies of the system. The internal energy, , expresses the thermodynamics of a system in the "energy-language", or in