Chemistry · General chemistry II · Worked example
Convert molarity to molality with density
A 2.00 M NaCl solution has a density of 1.08 g/mL. Find its molality.
Choose an amount
Take exactly 1.000 L of solution. At 2.00 M it holds 2.00 mol of NaCl.
Mass of the solution
The density turns the volume into a mass.
Mass of the solute
Multiply the moles by the molar mass.
Mass of the solvent
The rest of the solution is water: 963 g, or 0.963 kg.
Molality
Moles of solute per kilogram of solvent.
Compare the two
The molality is a little higher than the molarity because a liter of this solution holds less than a kilogram of water. In dilute water solutions the two are nearly equal.
Result
The molality is 2.08 m.
Your turn
A 1.50 M solution of ethanol, C₂H₅OH (46.07 g/mol), has a density of 0.985 g/mL. Find its molality.
Show the answer and explanation
1.64 m.
1.000 L weighs 985 g, of which 1.50 mol × 46.07 g/mol = 69.1 g is ethanol, leaving 916 g of water. 1.50 mol ÷ 0.916 kg = 1.64 m.
Keep exploring
Solutions & concentration finds the molality of 2.00 mol of NaCl in 963 g of water: 2.08 mol/kg.
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