Molality Calculator

How Molality Is Calculated

Molality is a concentration unit that expresses the number of moles of solute per kilogram of solvent (not solution). Unlike molarity, molality doesn’t change with temperature — making it preferred for colligative property calculations.

Molality Formula: m = n_solute / m_solvent(kg)

Where:

• m = molality in mol/kg (molal, symbol m)
• n_solute = moles of solute
• m_solvent = mass of solvent in kilograms (NOT total solution mass)

Moles of Solute: n = mass(g) / molar mass(g/mol)

Worked Example: Dissolve 58.44 g of NaCl (molar mass = 58.44 g/mol) in 750 g of water:

• n(NaCl) = 58.44 / 58.44 = 1.0 mol
• m_solvent = 750 g = 0.750 kg
• Molality = 1.0 / 0.750 = 1.333 mol/kg (1.333 m)

Colligative Property Applications:

• Boiling point elevation: ΔT_b = K_b × m (K_b for water = 0.512 °C/m)
• Freezing point depression: ΔT_f = K_f × m (K_f for water = 1.86 °C/m)

For the NaCl example above (dissociates into 2 ions, i = 2):

• Freezing point depression = 1.86 × 1.333 × 2 = 4.96°C → water freezes at −4.96°C

Molality vs Molarity:

• Molarity (M): mol/L solution — changes with temperature
• Molality (m): mol/kg solvent — temperature-independent
• For dilute aqueous solutions, M ≈ m (since 1L water ≈ 1 kg)