M

Molality

m = moles solute / kg solvent; units: mol/kg (molal)
Quick Reference
Formula / Notationm = moles solute / kg solvent; units: mol/kg (molal)
Also Known AsMolal concentration, m (molality), mol/kg, molality unit

What is Molality?

A concentration unit defined as the number of moles of solute per kilogram of solvent (mol/kg), symbol m. Unlike molarity, molality does not change with temperature since it is based on mass rather than volume. Used in colligative property calculations.

Formula & Notation

m = moles solute / kg solvent; units: mol/kg (molal)

Other Names / Synonyms: Molal concentration, m (molality), mol/kg, molality unit

Properties & Characteristics

Molality (m) is a concentration unit defined as the number of moles of solute per kilogram of solvent: m = n_solute / m_solvent (kg). Unlike molarity, molality is independent of temperature (since mass does not change with temperature) and is therefore preferred for colligative property calculations (boiling point elevation: ΔT_b = K_b × m; freezing point depression: ΔT_f = K_f × m). It is the appropriate concentration scale for thermodynamic calculations.

Uses & Applications

Molality is used in: colligative property calculations, activity coefficient expressions in electrochemistry (Debye-Hückel theory uses molality), thermodynamic calculations of solution properties, and in the pharmaceutical and food industry when precise temperature-independent concentrations are required.

Safety Information

No direct safety concern for the unit. Misunderstanding whether a concentration is expressed as molality or molarity can lead to preparation errors. For dilute aqueous solutions, molality ≈ molarity in numerical value, but for concentrated solutions (especially in non-aqueous solvents or dense solvents), significant differences arise.

Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.

Key Facts

Term Molality
Formula m = moles solute / kg solvent; units: mol/kg (molal)
Synonyms Molal concentration, m (molality), mol/kg, molality unit

Frequently Asked Questions

A concentration unit defined as the number of moles of solute per kilogram of solvent (mol/kg), symbol m. Unlike molarity, molality does not change with temperature since it is based on mass rather than volume. Used in colligative property calculations.

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