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Molality Calculator

Calculate molality, solute mass, solvent mass, or molar mass. Solve b = n / W with unit conversions and common solute reference.

Molality Worksheet
0.80082136
Molality0.80082136 mol/kg
Molality (b)
0.80082136
mol/kg
Moles (n)
0.20020534
mol
Solute Mass
11.7
gram [g]
Solvent Mass
250
gram [g]
Formula: b = (11.7 g / 58.44 g/mol) / 250 g = 0.80082136 mol/kg
Common Solute Molar Masses
SoluteFormulaMolar Mass (g/mol)Use
Sodium chlorideNaCl58.44
GlucoseC₆H₁₂O₆180.16
SucroseC₁₂H₂₂O₁₁342.3
Potassium chlorideKCl74.55
Sodium hydroxideNaOH40
Calcium chlorideCaCl₂110.98
Hydrochloric acidHCl36.46
Sulfuric acidH₂SO₄98.08
Acetic acidCH₃COOH60.05
UreaCO(NH₂)₂60.06
EthanolC₂H₅OH46.07
Potassium permanganateKMnO₄158.03

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The Molality Formula

Molality measures the concentration of a solution based on the mass of the solvent:

b = n / W

Where b is molality (mol/kg), n is moles of solute, and W is mass of solvent in kilograms. Since n = mass / molar mass:

b = msolute / (M × W)

  • Find molality: b = n / W
  • Find solute mass: m = b × M × W
  • Find solvent mass: W = n / b
  • Find molar mass: M = m / (b × W)

Molality vs. Molarity

PropertyMolality (b)Molarity (M)
Definitionmol solute / kg solventmol solute / L solution
Temperature dependentNoYes
Requires densityNoYes (for preparation)
Common useColligative propertiesLab concentrations

Colligative Properties

Molality is essential for calculating colligative properties — solution properties that depend on solute particle count rather than identity:

  • Boiling point elevation: ΔTb = Kb × b × i
  • Freezing point depression: ΔTf = Kf × b × i
  • Osmotic pressure: π = b × R × T × i (approximate)

Where Kb and Kf are solvent-specific constants, and i is the van 't Hoff factor (number of particles the solute dissociates into).

Common Solvent Constants

SolventKb (°C/m)Kf (°C/m)BP (°C)FP (°C)
Water0.5121.86100.00.0
Benzene2.535.1280.15.5
Acetic acid3.073.90118.116.6
Cyclohexane2.7920.080.76.6

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Frequently Asked Questions

What is the formula for molality?

Molality = moles of solute / kilograms of solvent, and its unit is mol/kg. Because moles = mass / molar mass, it can also be written as b = mass of solute / (molar mass × kg of solvent). The symbol is b or a lowercase m, though mol/kg is clearer because m can be mistaken for meters.

What is the difference between molality and molarity?

Molarity is moles of solute per liter of solution, while molality is moles of solute per kilogram of solvent. Molality does not change with temperature because mass does not change, whereas volume, and therefore molarity, does. That is why molality is used for boiling point elevation, freezing point depression and osmotic pressure.

What is the molality of 58.44 g of NaCl dissolved in 1 kg of water?

The molality is 1 mol/kg. NaCl has a molar mass of 58.44 g/mol, so 58.44 g is exactly 1 mole, and 1 mole divided by 1 kg of water gives 1 mol/kg. For colligative properties, multiply by the van t Hoff factor of 2, because each NaCl unit splits into Na+ and Cl- ions.

Do you use the mass of the solution or the solvent for molality?

Use the mass of the solvent only, in kilograms. Dividing by the mass of the whole solution, solute plus solvent, is the most common molality mistake and gives a value that is too low. If you only know the solution mass, subtract the solute mass first, then convert grams to kilograms.