Molality Calculator

Calculate molality (m), moles of solute, mass of solvent, or mass of solute. Molality is temperature-independent and ideal for colligative properties.

Core Formula: Molality (m) = moles of solute ÷ mass of solvent (kg) Related Formulas: Moles = Molality × Mass of Solvent (kg) Mass of Solvent (kg) = Moles ÷ Molality Moles = Mass of Solute (g) ÷ Molar Mass (g/mol) Mass of Solute (g) = Moles × Molar Mass (g/mol) Units: • Molality: mol/kg or m • Mass of Solvent: kg or g (1 kg = 1000 g) • Molar Mass: g/mol (from periodic table) Note: Molality uses mass of SOLVENT, not solution
Example 1 (Calculate Molality): Dissolve 58.44 g NaCl (MM = 58.44 g/mol) in 1 kg water Moles = 58.44 g ÷ 58.44 g/mol = 1 mol Molality = 1 mol ÷ 1 kg = 1 m Example 2 (Freezing Point Depression): Add 62.0 g ethylene glycol (C₂H₆O₂, MM = 62.07 g/mol) to 1 kg water Moles = 62.0 g ÷ 62.07 g/mol = 0.999 mol Molality = 0.999 mol ÷ 1 kg = 0.999 m ΔTf = Kf × m = 1.86 °C/m × 0.999 m = 1.86 °C Example 3 (Calculate Mass Needed): Prepare 2 m NaOH solution using 500 g water (MM = 40.00 g/mol) Mass solvent = 500 g = 0.5 kg Moles = 2 m × 0.5 kg = 1 mol Mass = 1 mol × 40.00 g/mol = 40.0 g Example 4 (Seawater Approximation): 35 g NaCl in ~1000 g water (MM NaCl = 58.44 g/mol) Moles = 35 g ÷ 58.44 g/mol = 0.599 mol Molality ≈ 0.599 mol ÷ 1 kg = 0.60 m

What is molality?

Molality (m) is the number of moles of solute per kilogram of solvent (not solution). It is written as mol/kg or m. Unlike molarity, molality is temperature-independent because mass does not change with temperature.

What is the difference between molality and molarity?

Molality uses kilograms of solvent in the denominator, while molarity uses liters of solution. Molality is preferred for colligative properties (boiling point elevation, freezing point depression) because it does not change with temperature.

How do I find the mass of solvent?

If you have the solution mass, subtract the solute mass: mass of solvent = mass of solution - mass of solute. For aqueous solutions, if you know the volume and assume density ≈ 1 g/mL, you can estimate: 1 L water ≈ 1 kg.

When should I use molality instead of molarity?

Use molality for colligative properties calculations (freezing point depression, boiling point elevation, osmotic pressure at varying temperatures), and in situations where temperature changes are significant. Use molarity for most other solution chemistry and reactions at constant temperature.

Can molality be greater than molarity?

Yes, typically molality is slightly larger than molarity for aqueous solutions because 1 kg of water has a volume less than 1 L when solute is added. The difference depends on the solution density.

How do I convert between molality and molarity?

You need the solution density (ρ in g/mL): M = m × ρ / (1 + m × MM/1000), where MM is molar mass in g/mol. The conversion depends on concentration and density, so there is no simple factor like with unit conversions.

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