Molality Calculator

Molality (m) is the number of moles of solute per kilogram of solvent, expressed in mol/kg. Molarity (M) is the number of moles of solute per liter of solution. The key difference is that molality uses the mass of the solvent and is temperature-independent, while molarity uses the volume of the solution and changes with temperature as the solution expands or contracts.

Use the formula: molality = moles of solute / mass of solvent (in kg). If you don't know the moles directly, divide the mass of solute (in grams) by its molar mass (g/mol) to get moles first. For example, 70.128 g of NaCl (molar mass 58.44 g/mol) = 1.2 mol. Dissolving this in 0.2 kg of water gives a molality of 6 mol/kg.

Molality = 3 mol / 0.5 kg = 6 mol/kg. Simply divide the number of moles of solute by the mass of the solvent in kilograms.

Use the formula: M = m × d / (1 + m × W), where M is molarity (mol/L), m is molality (mol/kg), d is the density of the solution (kg/L), and W is the molar mass of the solute (kg/mol). This conversion requires knowing the solution density, which is why molality and molarity are not directly interchangeable without additional information.

To convert molarity (M) to molality (m), you need the density of the solution (d in g/mL) and the molar mass of the solute (W in g/mol). The formula is: m = (M × 1000) / (d × 1000 − M × W). This accounts for the fact that molarity is based on solution volume while molality is based on solvent mass.

Molality is preferred for colligative property calculations — such as boiling point elevation, freezing point depression, and osmotic pressure — because it does not change with temperature or pressure. Since it is based on the mass of the solvent rather than the volume of the solution, it remains constant regardless of thermal expansion.

Molality is expressed in moles per kilogram (mol/kg), often written as 'm' or 'mol·kg⁻¹'. A 1 molal solution contains 1 mole of solute dissolved in 1 kilogram of solvent. Note that this differs from a 1 molar (1 M) solution, which contains 1 mole of solute per 1 liter of solution.

No — the solvent mass in the molality formula refers only to the pure solvent, not the total solution mass. This is a common mistake. If you know the total solution mass and the solute mass, subtract the solute mass from the solution mass to get the solvent mass before calculating molality.