Molality (m) is a measure of the concentration of a solute in a solution. It is defined as the number of moles of solute per kilogram of solvent (not the total solution).
Formula for Molality (m)
m=Moles of solute ÷ kg of solvent
Where:
- m = molality (mol/kg)
- moles of solute = amount of solute in moles.
- kilograms of solvent = mass of the solvent in kg.
Example Calculation
Calculate the molality of a solution prepared by dissolving 10.0 g of NaCl in 500 g (0.500 kg) of water.
Find moles of NaCl:
Molar mass of NaCl = 58.44 g/mol
Moles of NaCl = 10.0 g÷58.44 g/mol
=0.171 mol
Convert solvent mass to kg:
500 g = 0.500 kg
Calculate molality:
0.171 mol ÷ 0.500 kg =0.342 mol/kg
Thus, the molality of the solution is 0.342m.
Molality is commonly used in colligative properties calculations, such as boiling point elevation and freezing point depression.
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