Why molality is preferred over molarity in expressing the concentration of solution?

Molality (symbol: "m") and molarity (symbol: "M") are both ways to express the concentration of solute in a solution, but they have different advantages and applications. Molality is often preferred over molarity in certain situations due to its independence from temperature and volume changes.

Molality (m) is defined as the number of moles of solute per kilogram of solvent. It is a measure of the ratio of solute to solvent mass. Since it is based on mass, molality is not affected by changes in temperature or the volume of the solution, assuming the volume change doesn't cause a significant change in the mass of the solvent. This makes molality particularly useful in situations where temperature changes might occur, such as in reactions conducted at non-standard temperatures.

Molarity (M), on the other hand, is defined as the number of moles of solute per liter of solution. Molarity depends on both the amount of solute and the volume of the solution. As temperature changes, the volume of a liquid can change due to thermal expansion or contraction, which in turn affects the concentration of the solution when expressed in molarity. Also, changes in pressure (for gases dissolved in liquids) can lead to volume changes and affect the molarity of the solution.

Molality is especially important in cases where precise control over the concentration of a solution is necessary, such as in certain chemical reactions, freezing point depression measurements, and colligative property studies. When temperature changes are anticipated or volume changes are undesirable, molality provides a more accurate representation of the concentration of solute in a solution.

In summary, molality is preferred over molarity in expressing the concentration of a solution when temperature and volume changes need to be accounted for, as molality is not affected by these factors and provides a more consistent measure of concentration under varying conditions.