Molality Formula

Molality (m) measures concentration as moles of solute per kilogram of solvent. Unlike molarity, molality is temperature-independent and essential for colligative property calculations.

The Molality Formula

m = n / kgsolvent

Moles per kilogram of solvent

Variable Definitions

m = Molality

Units: mol/kg or m (lowercase m)

Concentration of solute

⚠️ Don't confuse lowercase m (molality) with uppercase M (molarity)

n = Moles of Solute

Units: mol

Formula: n = mass / molar mass

Amount of dissolved substance

kgsolvent = Mass of Solvent

Units: kg (kilograms)

Mass of SOLVENT only, NOT solution

⚠️ Must be in kg! Convert grams to kg by dividing by 1000

Molality vs Molarity

PropertyMolality (m)Molarity (M)
Formulamol / kg solventmol / L solution
Temperature EffectIndependentDependent
UsesColligative propertiesReactions, dilutions
DenominatorSolvent mass (kg)Solution volume (L)
Symbolm (lowercase)M (uppercase)

Rearranged Forms

Solve for moles:

n = m × kgsolvent

Solve for solvent mass:

kgsolvent = n / m

From mass of solute:

m = (gsolute / MM) / kgsolvent

Where MM = molar mass of solute

Step-by-Step Example

Problem: Calculate the molality when 23.0 g NaCl is dissolved in 500 g water. (NaCl MM = 58.44 g/mol)

Given:

  • Mass of NaCl (solute) = 23.0 g
  • Mass of water (solvent) = 500 g
  • Molar mass of NaCl = 58.44 g/mol

Step 1: Convert solvent mass to kg

kgsolvent = 500 g ÷ 1000 = 0.500 kg

Step 2: Calculate moles of solute

n = 23.0 g ÷ 58.44 g/mol = 0.394 mol

Step 3: Calculate molality

m = n / kgsolvent = 0.394 mol / 0.500 kg = 0.788 mol/kg

Answer: 0.788 m or 0.788 mol/kg

This solution has 0.788 moles of NaCl per kilogram of water.

Common Mistakes

❌ Using solution mass instead of solvent mass

Denominator is SOLVENT mass only, NOT total solution. Don't include the solute in the denominator!

❌ Forgetting to convert grams to kilograms

Solvent mass MUST be in kg. If given in g, divide by 1000. Missing this makes your answer 1000× too small!

❌ Confusing m (molality) with M (molarity)

Molality (m) = mol/kg solvent. Molarity (M) = mol/L solution. They're different units!

❌ Using grams instead of moles for solute

Must convert solute mass to moles using molar mass before calculating molality.

When to Use Molality

✅ Use Molality For:

  • • Boiling point elevation (ΔTb = Kb·m)
  • • Freezing point depression (ΔTf = Kf·m)
  • • Osmotic pressure calculations
  • • Any temperature-varying conditions
  • • Colligative properties

✓ Advantages:

  • • Temperature-independent
  • • Doesn't change with heating/cooling
  • • More accurate for non-aqueous solvents
  • • Easy to prepare without volumetric glassware

Related Calculators

Molality Calculator

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Molarity Calculator

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Concentration Converter

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Freezing Point Depression

Use molality for ΔTf

Related Calculators

Molality Calculator

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Freezing Point Depression

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Molarity vs Molality

Compare concentration units

Frequently Asked Questions

What is molality?

Molality (m) = moles of solute / kg of solvent. It's a concentration unit that doesn't change with temperature.

How is molality different from molarity?

Molality uses kg of solvent (mass), molarity uses liters of solution (volume). Molality is temperature-independent.

Why use kg instead of g?

Standard SI unit for molality is mol/kg. Using kg gives convenient numbers (usually 0.1-10 m range).

When should I use molality instead of molarity?

Use molality for colligative properties (boiling/freezing point changes) or when temperature varies significantly.

Can molality be greater than molarity?

Yes! For aqueous solutions, molality is usually slightly higher than molarity because 1 L water weighs less than 1 kg.

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