Molarity and molality are both important concepts in chemistry used to describe the concentration of a solution, but they differ in how they quantify this concentration.

Molarity, represented by the symbol $M$, is defined as the number of moles of solute (the substance being dissolved) per liter of total solution. This means that molarity takes into account the volume of the entire solution, which includes both the solute and the solvent (the substance in which the solute is dissolved). Mathematically, molarity can be expressed as:

where $n$ is the number of moles of solute and $V$ is the volume of the solution in liters.

Molality, on the other hand, is denoted by the symbol $m$ and is defined as the number of moles of solute per kilogram of solvent. This definition focuses solely on the mass of the solvent, ignoring the total volume of the solution. Therefore, molality is not influenced by changes in temperature or pressure, unlike molarity, which can vary if the volume of the solution changes due to such conditions. The formula for molality is given by:

where $n$ is the number of moles of solute and $m_s$ is the mass of the solvent in kilograms.

To illustrate the difference between the two, consider the following examples: If one mole of sugar is dissolved in one liter of water, the molarity of the resulting solution is $1 \, M$. Conversely, if one mole of sugar is dissolved in one kilogram of water, the molality of that solution is $1 \, m$.

In summary, while both molarity and molality are used to express the concentration of a solution, they do so in different ways. Molarity considers the total volume of the solution, whereas molality focuses solely on the mass of the solvent. This distinction makes molality a more reliable measure in situations where temperature and pressure may fluctuate, as it remains constant under such changes.