Molality Calculator

Calculate molality (m) in mol of solute per kg of solvent. Convert between molarity and molality. Essential for colligative property calculations: boiling point elevation and freezing point depression.

🧪 Solutions📐 m = n(solute)/kg(solvent)🧪 Chemistry
Moles of solute n (mol)
Mass of solvent (g)

Note: uses SOLVENT mass in grams. Convert grams to kg automatically.

Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Molality Calculator—m = n(solute)/kg(solvent)mol/kg

Step-by-Step Examples

Example 1
Basic Molality

0.500 mol NaCl dissolved in 500 g water.

  • m = n/kg = 0.500/(500/1000) = 0.500/0.500
  • m = 1.00 mol/kg
✓ m = 1.00 mol/kg
Example 2
From Mass

Dissolve 10.0 g NaOH (M=40.00) in 250 g water.

  • n(NaOH) = 10.0/40.00 = 0.250 mol
  • m = 0.250/(250/1000) = 0.250/0.250 = 1.00 mol/kg
✓ m = 1.00 mol/kg
Example 3
Dilute Solution

0.100 mol glucose in 900 g water.

  • m = 0.100/0.900 = 0.111 mol/kg
✓ m = 0.111 mol/kg

Real-World Applications

🧊
Colligative Properties
Molality used for boiling point elevation, freezing point depression, and osmotic pressure.
🌡️
Temperature-Independent Concentration
Unlike molarity, molality does not change with temperature (volume does not expand).
🧪
Physical Chemistry
Thermodynamic equations for solutions use molality to avoid temperature-volume complications.
☀️
Antifreeze Calculations
dTf = Kf x m x i. Molality directly plugged into colligative property equations.

Common Mistakes to Avoid

⚠️
Solvent mass, NOT solution mass

Molality uses kg of solvent (the substance dissolving into), not kg of total solution.

⚠️
Grams to kg conversion

500 g solvent = 0.500 kg. Must divide by 1000.

⚠️
Molality vs Molarity

Molality: mol/kg solvent. Molarity: mol/L solution. Different denominators. Approximately equal for dilute aqueous solutions.

Frequently Asked Questions

What is the difference between molality and molarity? ▾
Molarity: mol/L of solution (changes with T). Molality: mol/kg of solvent (independent of T). For water at 25 C and dilute solutions, approximately equal.
When should I use molality instead of molarity? ▾
For colligative properties (boiling/freezing point, osmotic pressure). For solutions at variable temperatures. For non-aqueous solvents where volume is hard to measure.
How do I convert molarity to molality? ▾
For dilute aqueous: m ~ M. Precisely: m = M/(density - M*M_solute/1000). Requires solution density.
What is a 1 molal solution? ▾
1 mol of solute in 1 kg of solvent. For water: 1 mol NaCl in 1000 g water (but note: total solution mass = 1058.44 g, different from 1 L).
What is the difference between solvent and solution? ▾
Solvent: the dissolving medium (water). Solution: solute + solvent together. Molality uses solvent mass. Molarity uses total solution volume.

Related Chemistry Calculators

🌡️ Boiling Point Elevation🧊 Freezing Point Depression💧 Osmotic Pressure⚗️ Mole Fraction🧪 Molarity

Formula Explorer connections

Interpretation: This formula connects solute amount and solution volume, mass or particle count to concentration and colligative behavior. Assumption: Distinguish solution volume from solvent volume, use the stated temperature, and account for dissociation or nonideal activity when required.

Molarity Calculator →Molarity Molality Converter →Mole Fraction Calculator →Chemistry Formula Explorer →