Knowledge database: 3.9 Molality

Molality is one of the less commonly used physical quantities in basic stoichiometry, but it is important to, at least, know the basic definition of molality. This quantity is defined as the amount of the dissolved substance (A) divided by the mass of the solvent (X) in which this substance is dissolved. Molality is nowadays represented by the letter b, although it can sometimes be represented by the letter m. The unit for molality is mol/kg.

The above mentioned definition can be represented by the formula shown above.

It is also interesting to mention that the value of molality in the case of aqueous solutions, is very close to the value of the molar concentration, because by taking into account the density of water, one liter of water (1000 ml), weighs about one kilogram (1000 g). However, in most other cases, this similarity of the mentioned physical quantities deviates more or less.