Molality

Under the molality (symbol: b ) is the quotient of the amount of substance (s) of a solute X and the mass (m ) of its solvent L. It is expressed in units of moles per kilogram:

With the Avogadro constant can be obtained from the molality calculate how many particles (atoms, ions, molecules, ...) of the substance X are dissolved in the observed mass of the solvent L. The molality is in contrast to the molarity ( molar concentration: moles of solute divided by the volume of solution ) is not temperature-dependent, since the mass does not change in contrast to volume with temperature. You can specify more accurate, since the masses can better determine when volumes. Nor can calculate the molality easier because you do not have to be taken into account when calculating a volume contraction (or dilation ). However, the molarity is in laboratory practice most significant.

Example

With a 1 molal aqueous solution of sodium chloride (NaCl), a solution is known consisting of one mole of NaCl (ie, about 58.4 grams), and one kg of water was prepared. The total weight of the solution is thus about 1058 grams.

The notation ... means - molal kilograms divided by moles. The observed particles (atoms, molecules, ...) must be specified exactly as always with the mole is used also in the indication of molalities, in the example: NaCl. The spelling is not recommended according to DIN standardization, because according to DIN 32625 ", the description, molal ' for the unit mol / kg no longer be used "; the DIN (German Institute for Standardization) has this standard but was withdrawn in April 2006 because there was no longer any need for it.