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Molality Calculator

Chemistry

Calculate the molality of a solution from the mass of solute, molar mass, and mass of solvent. Get accurate mol/kg results with full step-by-step working.

58.44 g
g
58.44 g/mol
g/mol
1,000 g
g

Molality (mol/kg)

1
Moles of Solute (mol)
1
Solvent Mass (kg)
1

This calculator computes your Molality (mol/kg), Moles of Solute (mol), Solvent Mass (kg) from the values you enter.

Inputs
Mass of SoluteMolar Mass of SoluteMass of Solvent
Outputs
Molality (mol/kg)Moles of Solute (mol)Solvent Mass (kg)

What is a Molality?

The Molality Calculator determines the molal concentration of a solution — the number of moles of solute present per kilogram of solvent. Molality (symbol m, unit mol/kg) is a fundamental concentration measure in physical chemistry, particularly for any calculation that involves a change in temperature. While molarity depends on the volume of solution and therefore shifts as temperature rises or falls, molality is fixed by mass, which does not change. This makes it the preferred concentration unit for colligative property calculations.

Colligative properties — boiling point elevation, freezing point depression, osmotic pressure, and vapour pressure lowering — all depend on the number of solute particles relative to the amount of solvent, not on the volume of the mixture. Each of these calculations uses molality as the concentration input, with constants specific to the solvent (Kb for boiling point elevation, Kf for freezing point depression).

In Indian Class 12 Chemistry (NCERT Solutions Chapter), molality appears alongside molarity and mole fraction as one of the three principal concentration units. JEE and NEET questions on solutions almost always require students to switch between these units and apply the appropriate one for the property being calculated. Confusing molality with molarity is one of the most common errors in this topic, and this calculator helps students verify which value they have computed.

Beyond academics, molality is used in antifreeze and de-icing formulations, cryoscopic molecular weight determination, and food science, where solutions must behave predictably across a range of temperatures. The Molality Calculator works for any solute-solvent pair — simply enter the mass of solute, its molar mass, and the mass of solvent to get the molality instantly.

How to use this Molality calculator

  1. Enter the Mass of Solute — type the mass of your dissolved substance into the Mass of Solute field, in grams (g). For example, for NaCl dissolved in water, enter the number of grams weighed out.
  2. Enter the Molar Mass of Solute — type the molar mass of the solute in the Molar Mass of Solute field, in g/mol. For NaCl this is 58.44 g/mol; look up values from the periodic table or a chemistry data book for other compounds.
  3. Enter the Mass of Solvent — type the mass of the pure solvent in the Mass of Solvent field, in grams (g). This is the mass of solvent only, not the total solution mass. For 1 litre of water, enter 1000 g.
  4. Read the results — the calculator displays Molality (mol/kg) as the highlighted result, along with Moles of Solute (mol) and Solvent Mass (kg) as supporting outputs.
  5. Verify the solvent mass conversion — check the Solvent Mass (kg) output to confirm the gram-to-kilogram conversion was applied correctly.
  6. Use the molality downstream — apply the molality value in colligative property formulas: ΔTb = Kb × m (boiling point elevation) or ΔTf = Kf × m (freezing point depression), substituting the Kb or Kf constant for your specific solvent.

Formula & Methodology

Molality is calculated in two steps:

Step 1 — Convert solute mass to moles:

> n = m_solute ÷ M_r

Where:
- n = moles of solute (mol)
- m_solute = mass of solute (g)
- M_r = molar mass of solute (g/mol)

Step 2 — Convert solvent mass to kg and calculate molality:

> molality = n ÷ (m_solvent ÷ 1000)

Where:
- molality = molal concentration (mol/kg)
- m_solvent = mass of solvent (g)
- m_solvent ÷ 1000 converts grams to kilograms

Combined formula:

> molality = m_solute ÷ (M_r × (m_solvent ÷ 1000))

Worked example:

Dissolve 90 g of glucose (C₆H₁₂O₆, molar mass = 180.16 g/mol) in 500 g of water.

- n = 90 g ÷ 180.16 g/mol = 0.4996 mol ≈ 0.5 mol
- Solvent mass = 500 g ÷ 1000 = 0.5 kg
- Molality = 0.5 mol ÷ 0.5 kg = 1.0 mol/kg

This 1 molal glucose solution would elevate the boiling point of water by Kb × m = 0.512 × 1.0 = 0.512 °C, raising it from 100 °C to approximately 100.51 °C at standard pressure.

Frequently Asked Questions

Molality is a concentration unit defined as the number of moles of solute dissolved per kilogram of solvent. It is represented by the symbol m (lowercase) and has the unit mol/kg. Unlike molarity, molality does not depend on the volume of the solution, which means it remains constant regardless of temperature or pressure changes.
The formula for molality is m = n ÷ kg(solvent), where m is the molality in mol/kg, n is the number of moles of solute, and kg(solvent) is the mass of the solvent in kilograms. Since the number of moles is derived from the solute's mass, the full formula is m = mass(g) ÷ (molar mass(g/mol) × mass of solvent(kg)).
Molarity (M) is expressed as moles of solute per litre of solution and changes with temperature because liquid volumes expand and contract with heat. Molality (m) is expressed as moles of solute per kilogram of solvent and is temperature-independent because mass does not change with temperature. For colligative property calculations — such as boiling point elevation and freezing point depression — molality is the correct concentration unit to use.
Use molality whenever the experiment involves temperature changes, such as calculating the boiling point elevation or freezing point depression of a solution. Because the mass of a solvent does not change with temperature (unlike its volume), molality gives consistent results across different temperatures. The [Molarity Calculator](/molarity-calculator/) is more appropriate when preparing standard solutions at a fixed temperature.
Molality measures moles of solute per kilogram of solvent, while mole fraction measures the ratio of moles of one component to the total moles of all components. Mole fraction is dimensionless and is used in Raoult's law and vapour pressure calculations. Molality is used for colligative properties such as boiling point elevation and osmotic pressure, while mole fraction is preferred in thermodynamics.
To find moles, divide the mass of the solute in grams by its molar mass in g/mol. For example, 58.44 g of NaCl divided by a molar mass of 58.44 g/mol gives 1 mole. The Molality Calculator performs this automatically from the Mass of Solute and Molar Mass of Solute inputs. If you need a standalone conversion, use the [Grams to Moles Calculator](/grams-to-moles-calculator/).
The solvent is the substance that dissolves the solute — typically water in most chemistry contexts. The solution is the final mixture of solute and solvent together. When calculating molality, you use the mass of solvent alone (not the total mass of the solution), which is an important distinction. The total solution mass equals the solvent mass plus the solute mass.
If you know the total mass of the solution and the mass of solute, subtract the solute mass from the solution mass to get the solvent mass: mass of solvent = mass of solution − mass of solute. For example, if 1050 g of solution contains 50 g of dissolved NaCl, the solvent mass is 1050 − 50 = 1000 g. Enter this value (in grams) into the Mass of Solvent field.
Yes, molality has the unit mol/kg, sometimes written as mol·kg⁻¹ or expressed verbally as 'molal'. A solution with a molality of 1 mol/kg is called a 1 molal solution. In everyday lab practice the unit is often abbreviated as simply m — for example, a '0.5 m NaCl solution' means 0.5 moles of NaCl per kilogram of water.
Molality is taught in Class 12 Chemistry (NCERT Chapter 2 — Solutions) and is a standard topic in JEE Main, JEE Advanced, and NEET preparation. Questions on boiling point elevation, freezing point depression, and osmotic pressure all require molality. Indian university practical courses also use molality when working with solutions at variable temperatures, particularly in thermochemistry and colligative property experiments.
Molality and molarity are approximately equal for very dilute aqueous solutions because the density of dilute water solutions is close to 1 kg/L, so 1 litre of solution weighs approximately 1 kg of solvent. As concentration increases, the two values diverge significantly. For concentrated solutions, always use the appropriate unit for the application — molality for temperature-sensitive work and molarity for volumetric stoichiometry.
The identity of the solvent does not appear in the molality formula itself — only its mass in kilograms matters. However, the solvent determines the colligative properties (boiling point elevation constant Kb, freezing point depression constant Kf) that you use downstream in calculations. For aqueous solutions, water is the solvent; in organic chemistry, solvents such as benzene or ethanol may be used, each with their own Kb and Kf values.
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