Question: When is molality used instead of molarity? Why is it used?
Answer: Molality (m) and molarity (M) both express the concentration of a chemical solution. Molality is the number of moles of solute per kilogram of solvent. Molarity is the number of moles of solute per liter of solution. If the solvent is water and the concentration of solute is fairly low (i.e., dilute solution), molality and molarity are approximately the same. However, the approximation fails as a solution becomes more concentrated, involves a solvent other than water, or if it undergoes temperature changes that could change the density of the solvent. In this situations, molality is the preferred method of expressing concentration because the mass of solute and solvent in a solution does not change.
In particular, molality is used when you:
- Determine boiling point
- Determine melting point
- Work with colligative properties (boiling point elevation, freezing point depression)
Use molality any time you expect the solute may interact with the solute. Use molarity for dilute aqueous solutions held at a constant temperature. In general, the difference between molarity and molality for aqueous solutions near room temperature is very small and it won't make a difference whether you use a molar or molal concentration.
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