Molality Calculator
This molality calculator finds the amount of solute per kilogram of solvent. Example: 58.44 g of table salt in 1000 g of water give 1 mol/kg.
Example: What is given: Mass and molar mass of the solute · Mass of the solute 58.44 g · Molar mass M 58.44 g/mol · Mass of the solvent 1,000 g → Result: Molality b = 1 mol/kg (1 mol of solute in 1 kg of solvent). Source: IUPAC Green Book – Quantities, Units and Symbols in Physical Chemistry (Molalität b_B = n_B / m_A). Updated: .
How it is calculated
Molality b tells you how many moles of a substance are dissolved in one kilogram of solvent. It is given in mol/kg (IUPAC: b_B = n_B / m_A). Unlike molarity, which refers to the volume of the solution, molality does not depend on temperature, because masses do not change when heated.
Formula
From the mass m of the solute and its molar mass M you get the amount n = m / M. The molality is then b = n / m(solvent), with the solvent mass in kilograms. The calculator also shows the mass fraction w = m(solute) / (m(solute) + m(solvent)).
Worked example
58.44 g of sodium chloride (M = 58.44 g/mol) is dissolved in 1000 g of water. The amount is 58.44 / 58.44 = 1 mol, the solvent weighs 1 kg, so b = 1 mol/kg. The mass fraction is 58.44 / 1058.44 ≈ 5.52%. If 0.5 mol is dissolved directly in 250 g of solvent, b = 0.5 / 0.25 = 2 mol/kg.
- Molality is the basic quantity for boiling-point elevation and freezing-point depression.
- Molality and molarity of dilute solutions are similar only for solvents of about 1 kg/L (water: about 1 kg/L at room temperature).
- Look up the molar mass in the periodic table (IUPAC atomic weights).
Frequently asked questions
How do you calculate molality?
Divide the amount of the solute by the mass of the solvent in kilograms: b = n / m. In the example 1 mol / 1 kg = 1 mol/kg.
What is the difference between molality and molarity?
Molality refers to the mass of the solvent (mol/kg), molarity to the volume of the solution (mol/L). Only molality is independent of temperature.
Does the denominator include the mass of the whole solution?
No, only the mass of the solvent. Do not add the mass of the solute to the solvent mass.
Sources and legal basis
- IUPAC Green Book – Quantities, Units and Symbols in Physical Chemistry (Molalität b_B = n_B / m_A)
- IUPAC – Atomgewichte (Na 22,990, Cl 35,45 → NaCl 58,44 g/mol)
- Wikipedia – Molality
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