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Molality Calculator

Find the molality (mol/kg) of a solution.

How it is calculated

The tool divides the amount of solute by the mass of the solvent: m = n ÷ w, where n is the solute in moles and w is the solvent mass in kilograms. The answer is in mol/kg with four decimals.

Moles can be zero, which gives 0, but cannot be negative. The solvent mass must be greater than zero. Only the solvent counts in the denominator, not the whole solution: the mass of the dissolved substance is left out. To get moles from a weighed amount, divide the mass in grams by the molar mass in g/mol. The tool does not do this step, and it does not convert grams to kilograms, so a solvent given in grams must be divided by 1000 first.

Worked example

29.22 g of sodium chloride (molar mass 58.44 g/mol) is dissolved in 750 g of water.

Moles of solute: 29.22 ÷ 58.44 = 0.5 mol. Solvent mass: 750 g = 0.75 kg. Molality: 0.5 ÷ 0.75 = 0.6667 mol/kg. Note that this uses the water alone, not the 779.22 g of the finished solution.

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Common mistakes

  • Typing the solvent mass in grams. 750 g must be entered as 0.75 kg, otherwise the result is 1000 times too small.
  • Using the mass of the whole solution in place of the solvent. Molality counts only the solvent.
  • Entering grams of solute instead of moles. Convert with the molar mass first.
  • Confusing molality with molarity. Molarity divides by litres of solution and changes with temperature; molality divides by kilograms of solvent.
  • Using the solvent's volume in litres and assuming 1 L equals 1 kg. That only holds for water near room temperature, and not for other solvents.

About this calculator

Molality measures solution concentration as m = moles of solute / kilograms of solvent (units mol/kg, sometimes called "molal"). Because it's defined by mass of solvent rather than volume of solution, molality doesn't shift with temperature the way molarity does (volume expands or contracts as liquids heat and cool), making it the preferred unit whenever precise, temperature-independent concentration matters.

Physical chemists rely on molality for colligative-property calculations — freezing-point depression (ΔTf = Kf·m·i) and boiling-point elevation (ΔTb = Kb·m·i) — where the constants Kf and Kb are only valid when concentration is expressed this way. Lab chemists preparing standard solutions for cryoscopy or ebullioscopy experiments, and students working through physical chemistry problem sets, use molality for exactly this reason.

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Comments (1)

  • Editorial team

    Molality is the number to use when temperature changes, for example in freezing-point and boiling-point calculations. Take the 0.6667 mol/kg salt solution from the example: boiling-point rise is i × 0.512 × m for water, so with i = 2 for fully dissociated NaCl you would expect roughly 0.68 °C. That is an ideal-solution estimate. Which solute and solvent are you working with?

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