October 6, 2025

What Is Molality

Q: What is the key difference between molality and molarity?

Molality (moles of solute per kg of solvent) is temperature-independent because it uses mass, while molarity (moles of solute per liter of solution) is temperature-dependent because volume changes with temperature.

Q: When is molality preferred over molarity?

Molality is preferred in studies involving colligative properties (e.g., freezing point depression) and experiments where temperature and pressure variations might affect the solution's volume, as it provides a more consistent concentration measure.

Q: Can molality be greater than molarity?

Yes, depending on the density of the solvent and solution. For aqueous solutions, especially dilute ones, molality and molarity are often similar. However, if the solution density is less than 1 g/mL, molality can be greater than molarity.

Q: Is molality affected by the type of solvent?

While the *value* of molality depends only on the moles of solute and mass of the specific solvent used, the *properties* and behavior of the solution (like colligative properties) are inherently linked to the specific solvent's characteristics.

Discover molality, a measure of solute concentration, and understand its definition, formula, units, and why it's preferred in certain chemical contexts over molarity.

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Defining Molality (m)

Molality (symbolized as 'm') is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. Unlike molarity, which uses the total volume of the solution, molality focuses solely on the mass of the solvent, making it independent of temperature and pressure changes.

The Molality Formula and Units

The formula for calculating molality is: m = (moles of solute) / (mass of solvent in kilograms). The SI unit for molality is moles per kilogram (mol/kg). A solution with a molality of 1 mol/kg is sometimes referred to as '1 molal' or '1 m'.

Practical Example of Molality Calculation

If you dissolve 0.5 moles of glucose in 1 kilogram of water, the molality of the solution would be 0.5 mol/kg. If you instead dissolved 1 mole of sodium chloride in 500 grams (0.5 kg) of water, the molality would be 1 mol / 0.5 kg = 2 mol/kg.

Importance and Applications of Molality

Molality is particularly useful in experiments where temperature changes are significant, as the volume of a solution can expand or contract, altering its molarity, while the mass of the solvent remains constant. It is crucial in colligative properties calculations (like boiling point elevation, freezing point depression, and osmotic pressure), where the number of solute particles relative to the solvent mass is critical.

Frequently Asked Questions

What is the key difference between molality and molarity?

When is molality preferred over molarity?

Can molality be greater than molarity?

Is molality affected by the type of solvent?