Is osmolality preferred over osmolarity?

It is, indeed.

Colligative properties, which rely only on the quantity of solute particles in a unitary volume of solution (molarity) or mass of solvent (molality), are such as freezing point depression, osmotic pressure, and vapour pressure. These properties are independent of the type and mass of solute particles.

The measure of a colligative property is a collective measure of all the moles/molecules of solute in addition to a direct measure of molar or molal concentration, since a colligative property is directly proportional to the concentration of all particles of all the different solutes in ideal solutions.

Since osmotic pressure is the most reasonable and consistent measurable property at these low concentrations, only very dilute solutions behave optimally. For this reason, the term "osmotic" refers to the total osmoles of all solutes, even though these have typically been determined from freezing point depression measurements of solutions.

In order to comprehend how osmolality or osmolarity is produced, we must first measure a colligative-collective property for a given complex solution (such as blood plasma, milk, serum, etc.). Next, we calculate a molarity (moles per liter of solution) or osmolality (moles per kilogram of solvent) based on the experimental measure, assuming that the solution is ideal. Finally, we refer to the computed value as osmolality or osmolarity.

Osmolality for aqueous solutions is slightly higher than osmolarity because the latter includes a little bit more volume (due to solutes) in the denominator, reducing the ratio osmoles/volume (L) with respect to osmoles/mass (kg) for the same osmoles of solute. Since solute moles and solvent mass are unaffected by pressure and temperature, the molality is also unaffected by these parameters, making it the preferred expression.

In any case, because real solutions behave non-ideally, even though osmolality is preferred, both osmolality and osmolarity, measured for solutions in which the concentration and nature of each solute are precisely known, differ from the actual total molality and molarity (i.e., calculated osmolality and osmolarity). This means that the measured osmolalities, as the osmolarities, cannot be taken as the true values of total molality or molarity.

Yes, osmolality is often preferred over osmolarity in clinical settings because it is less affected by temperature and the volume changes of the solution. Osmolality measures the number of osmoles of solute per kilogram of solvent, making it a concentration independent of volume and temperature. Osmolarity, on the other hand, measures the number of osmoles per liter of solution, which can vary with temperature and pressure. Thus, osmolality provides a more accurate and reliable measure of the solute concentration in body fluids, important for diagnosing and treating osmotic balance disorders.