Skip to content

Molality Calculator

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

mol/kg
g/mol
Molality
0.80082136 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

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

Related Calculators

Percentage Calculator

Calculate percentages instantly. Find what percent of a number is, percentage increase/decrease, and more.

Fraction Calculator

Add, subtract, multiply, and divide fractions. Get results as simplified fractions, mixed numbers, and decimals.

Equivalent Fractions Calculator

Find equivalent fractions for any proper, improper, or mixed fraction. Shows the first 100 equivalent fractions.

Average Calculator

Calculate mean, median, mode, range, and standard deviation. Complete statistics for any data set.

Ratio Calculator

Simplify ratios, solve proportions, and scale ratios. Convert between ratios, decimals, and percentages.

Exponent Calculator

Calculate powers, roots, exponentials, and logarithms. Solve xⁿ, ⁿ√x, eˣ, and log expressions.

Frequently Asked Questions

What is molality?

Molality (b or m) measures the concentration of a solution as moles of solute per kilogram of solvent. Unlike molarity, molality does not change with temperature because it is based on mass rather than volume.

What is the molality formula?

Molality = moles of solute / mass of solvent (in kg). Since moles = mass / molar mass, it can also be written as b = mass_solute / (molar_mass × mass_solvent_kg).

What is the difference between molality and molarity?

Molarity (M) is moles of solute per liter of solution, while molality (b) is moles of solute per kilogram of solvent. Molality is temperature-independent because mass does not change with temperature, whereas volume (and thus molarity) does.

When should I use molality instead of molarity?

Molality is preferred for colligative property calculations (boiling point elevation, freezing point depression, osmotic pressure) and when working across a range of temperatures, since it remains constant regardless of thermal expansion.

What are the units of molality?

The SI unit of molality is mol/kg (moles per kilogram of solvent). It is sometimes written as "m" (lowercase), though this can be confused with meters, so mol/kg is more precise.

How do I convert molality to molarity?

M = (b × ρ) / (1 + b × M_solute / 1000), where ρ is the solution density in kg/L and M_solute is the molar mass of the solute in g/mol. You need to know the solution density for this conversion.