Molality Calculator

The Molality Calculator finds the molality (m) of a solution — the number of moles of solute per kilogram of solvent — and then shows you why that unit matters. Enter the solute as moles, or as grams plus its molar mass, give the mass of solvent, and the tool returns the molality in mol/kg along with the freezing-point depression and boiling-point elevation your solution would produce. Because molality is defined from the mass of solvent, it never changes with temperature, which is exactly why it is the concentration unit used for colligative properties.

What Is Molality?

Molality, given the symbol m, is the amount of solute (in moles) dissolved per kilogram of solvent. It is one of the core ways chemists express concentration. Its defining feature is that it depends only on mass, not on volume — so heating or cooling a solution does not change its molality. This makes molality the unit of choice for the colligative property equations that predict how a dissolved solute shifts the freezing and boiling points of a solvent.

Molality Formula

If you only know the mass of solute, first convert it to moles using the molar mass, then apply the molality formula:

How Molality Drives Colligative Properties

The two most common colligative properties — freezing-point depression and boiling-point elevation — are calculated directly from molality:

Here \( K_f \) and \( K_b \) are constants specific to each solvent, and \( i \) is the van't Hoff factor — the number of particles each formula unit of solute releases when it dissolves. This is the chemistry behind salting icy roads (lowering water's freezing point) and adding antifreeze to a car radiator (raising the boiling point).

Molality vs Molarity

Molality and molarity sound almost identical and are the single most confused pair in introductory chemistry. The key difference is the denominator:

Common Solvent Constants

Worked Example

Dissolve 0.5 mol of glucose (a non-electrolyte, so \( i = 1 \)) in 250 g of water:

• Convert solvent mass: 250 g = 0.25 kg
• Molality: \( m = 0.5 \div 0.25 = 2.0\;m \)
• Freezing-point depression: \( \Delta T_f = 1 \times 1.86 \times 2.0 = 3.72\,°C \), so the solution freezes at −3.72 °C
• Boiling-point elevation: \( \Delta T_b = 1 \times 0.512 \times 2.0 = 1.024\,°C \), so it boils at 101.02 °C

What Affects the Result?

How to Use This Calculator

1. Enter your solute: Type the moles of solute directly, or switch to the "mass + molar mass" tab and enter grams with the molar mass in g/mol.
2. Enter the solvent mass: Provide the mass of solvent and pick the unit (g, kg, or mg). Use solvent mass only, not solution mass.
3. Choose solvent and solute type: Select the solvent (water by default) and the van't Hoff factor that matches your solute.
4. Click Calculate: Read the molality, the freezing/boiling shifts on the temperature bar, and the full step-by-step breakdown.

Frequently Asked Questions

What is molality?

Molality (m) is a measure of concentration equal to the number of moles of solute divided by the mass of solvent in kilograms. Its unit is mol/kg, written as a lowercase italic m. Because it is based on mass rather than volume, molality does not change with temperature.

How do you calculate molality?

Divide the moles of solute by the mass of the solvent in kilograms: molality = moles of solute ÷ kilograms of solvent. If you know the solute mass in grams instead of moles, first divide the mass by the molar mass to get the moles, then divide by the kilograms of solvent.

What is the difference between molality and molarity?

Molarity (M) is moles of solute per litre of solution, while molality (m) is moles of solute per kilogram of solvent. Molarity depends on volume, which changes with temperature, so molarity is temperature-dependent. Molality is based on mass, so it stays constant at any temperature, which is why molality is used for colligative property calculations.

Why is molality used for freezing point and boiling point?

Freezing-point depression and boiling-point elevation are colligative properties that depend on the ratio of solute particles to solvent. Because they are measured across a range of temperatures, the concentration unit must not change with temperature. Molality, being mass-based, is temperature-independent, so it gives consistent results in the equations ΔTf = i × Kf × m and ΔTb = i × Kb × m.

What is the van't Hoff factor?

The van't Hoff factor i is the number of dissolved particles produced per formula unit of solute. Non-electrolytes such as sugar have i = 1, NaCl has i = 2 because it splits into Na⁺ and Cl⁻, and CaCl₂ has i = 3. A larger i means a stronger effect on freezing point, boiling point, and other colligative properties.

Does temperature affect molality?

No. Molality is defined using the mass of the solvent, and mass does not change with temperature. This is the main advantage of molality over molarity, whose value shifts as a solution expands or contracts when heated or cooled.

Additional Resources

• Molality - Wikipedia
• Colligative Properties - Wikipedia