Question

Molality is expressed in

Answer 1

Molality is directly related to mass of the solvent. SI unit of number of moles of solute (n) is mol and SI unit of weight of solvent is Kg. Formula for molality is $m = \dfrac{n}{w}$ , where m is molality, n is number of moles of solute and w is weight of solvent in kilogram.

Complete step-by-step answer: This question belongs to some basic concepts of chemistry. Let us see what molality actually means.
Definition of molality as we know is,
Molality is a property of solution. It is defined as the number of moles of solute present in $1000g$ of solvent. In short it is used to express the concentration of solute. In case if the solution contains more than one solvent then the molality is considered as pure pseudo-solvent. It is denoted by ‘m’ .
Let us consider an example: $1$ molal of $NaCl$ means $1$ mol of $NaCl$ dissolved in $1$ Kg of solvent.
Molality does not change with change in temperature.
The advantage of using molality is that molality depends on the mass of solute and mass of solvent only and mass remains unaffected by temperature and pressure. Another advantage of molality is that molality of one solute in a solution is independent of the presence of another solute.
Mathematically molality is
$m = \dfrac{n}{w}$ , where m is molality, n is number of moles of solute and w is weight of solvent in kilogram.
Hence, it is expressed in moles per kilograms or $mol$ $k{g^{ - 1}}$ .

Note: Remember that we are using weight of the solvent and not of the solution to calculate molality. Molality is independent of temperature of the solution. The terms molality and molarity sound the same but differ significantly in their formulas and definition.