The "Molality of Solute" of a solution is defined as the amount of substance of solute divided by the mass in kg of the solvent.
$b_B = \frac{n_B}{m_a}$, where $n_B$ is the amount of substance and $m_A$ is the mass.
Applicable units are those of quantitykind:MolalityOfSolute
Molality of Solute