Molarity to molality
Under SOLUTION, it is possible to define concentrations of elements (or valences of elements) in mol/L (molarity) (-units mol/L).
The molarity of NaCl (CNaCl) is converted to molality (mNaCl) through the following formula (CRC Handbook of Chemistry and Physics) :
$$ m_{NaCl} = \frac{1000 \cdot C_{NaCl}}{1000 \cdot \rho - (C_{NaCl} \cdot M_{NaCl})} $$
, where ρ and MNaCl are respectively the solution density and the NaCl molar mass (in g/mol). The solution density is thus needed to convert salt molarity to molality. When density measurement is available, it can be entered with -density (in g/cm3) ; this is this value that will be used in the preceding formula to convert molarity to molality. If the density measurement is not available, I recommend to use the -density 1 calculate option (with a database containing -Vm's), especially when the solution is concentrated (see below).
In the Figure below, are compared the calculated (outputted) molalities from inputted molarities of n NaCl solutions with firstly the density 1 calculate option disabled ("_rho-calc_OFF" label in the below graph):
SOLUTION n
-units mol/L
Na x
Cl x
# density 1 calculate
SOLUTION n
-units mol/L
Na x
Cl x
density 1 calculate
x the inputted NaCl molarity, is varying from 0.1 to 4.5. From [NaCl] = 1 mol/L, the -density 1 calculate option needs to be enabled in order to make the outputted molalities fit the experimental data (Table A-4 of Rogers and Pitzer, 1982)).
