Molality Calculator
Calculate molality (mol solute per kg solvent) and mole fraction.
Convert between molality and molarity using solution density.
Essential for colligative properties.
Molality (m) is defined as moles of solute per kilogram of solvent (not solution).
m = n_solute / kg_solvent = (mass_solute / M_solute) / kg_solvent
Units: mol/kg (sometimes written as molal, abbreviated mol kg⁻¹)
Why use molality instead of molarity? Molarity (M, mol/L) changes with temperature because volume changes. Molality is temperature-independent (mass doesn’t change with temperature). This makes molality preferred for colligative property calculations (boiling point elevation, freezing point depression, osmotic pressure).
Mole fraction:
x_solute = n_solute / (n_solute + n_solvent)
x_solvent = n_solvent / (n_solute + n_solvent)
Note: x_solute + x_solvent = 1 (for a two-component system).
Converting molality to molarity:
M (mol/L) = (m × ρ) / (1 + m × M_solute/1000)
where ρ = solution density (g/mL).
Key relationships for dilute aqueous solutions (ρ ≈ 1 g/mL, M_solvent = 18 g/mol):
- At low concentrations, M ≈ m (within a few percent)
- Mole fraction: x_solute ≈ m × 0.018 (for dilute solutions in water)
Example: A 1.00 mol NaCl in 1.00 kg water:
- Molality = 1.00 mol/kg
- Mole fraction NaCl = 1.00 / (1.00 + 1000/18) = 1.00/56.6 = 0.01768
- Mole fraction water = 0.9823