Molality Calculator
Formula: b = n / m
What is Molality?
Molality describes the amount of substance per kilogram of solvent.
Definition:
b = n / msolvent
b = n / msolvent
Unlike molarity, molality is temperature-independent because no volume is used.
- Unit: mol/kg
- Symbol: b (often also m)
- Application: colligative properties
Temperature-independent concentration unit
Formulas
Molality:
b = n / msolvent
b = n / msolvent
Amount of substance:
n = msubstance / M
n = msubstance / M
Combined formula:
b = (msubstance/M) / msolvent
b = (msubstance/M) / msolvent
Unit:
[b] = mol · kg-1
[b] = mol · kg-1
Examples
NaCl in water
5.84 g NaCl, M = 58.44 g/mol, 1 kg water
n = 5.84 / 58.44 = 0.1 mol
b = 0.1 / 1 = 0.1 mol/kg
5.84 g NaCl, M = 58.44 g/mol, 1 kg water
n = 5.84 / 58.44 = 0.1 mol
b = 0.1 / 1 = 0.1 mol/kg
Sugar solution
34.2 g sucrose, M = 342 g/mol, 0.5 kg water
n = 0.1 mol, b = 0.1 / 0.5 = 0.2 mol/kg
34.2 g sucrose, M = 342 g/mol, 0.5 kg water
n = 0.1 mol, b = 0.1 / 0.5 = 0.2 mol/kg
Direct from amount of substance
n = 0.3 mol in 2.0 kg water
b = 0.3 / 2.0 = 0.15 mol/kg
n = 0.3 mol in 2.0 kg water
b = 0.3 / 2.0 = 0.15 mol/kg
Ethylene glycol solution
n = 1.2 mol in 3.0 kg solvent
b = 1.2 / 3.0 = 0.4 mol/kg
n = 1.2 mol in 3.0 kg solvent
b = 1.2 / 3.0 = 0.4 mol/kg
Technical Background
Molality vs. Molarity
Molarity (mol/L) depends on volume and is therefore temperature-dependent. Molality uses masses and remains constant with temperature changes.
Typical applications
- Freezing point depression: ΔTf = Kf · b
- Boiling point elevation: ΔTb = Kb · b
- Osmotic pressure: proportional to particle number
- Physical chemistry: preferred concentration unit
Advantages
- Independent of temperature and pressure
- Robust for concentrated solutions
- Direct link between amount of substance and mass
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