What is the difference between molarity and molality, and why does molality not change with temperature?
GradeHunt_TeamGradeHunt Team · · Gujarat Board 12th
I'm in GSEB 12th Science, Chemistry chapter Solutions. Molarity and molality sound the same and I keep using volume where mass should come. Also the book says molality is independent of temperature but molarity is not. Can someone explain with a numerical?
Look at what sits in the denominator. That is the whole difference.
Molarity (M) = moles of solute / volume of solution in litres
Unit: mol L^-1
Molality (m) = moles of solute / mass of solvent in kg
Unit: mol kg^-1
Note two things. Molarity uses the whole solution. Molality uses only the solvent. And molarity uses volume while molality uses mass.
Why temperature matters: when a solution is heated, it expands, so its volume increases. The moles of solute stay the same, so molarity goes down. Mass does not change on heating, so molality stays the same. This is why molality is preferred in colligative properties like elevation of boiling point and depression of freezing point, where the temperature changes.
Worked example 1 (molarity)
5.85 g of NaCl is dissolved in water and the solution is made up to 250 mL.
Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol
Moles = 5.85 / 58.5 = 0.1 mol
Volume = 250 mL = 0.250 L
Molarity = 0.1 / 0.250 = 0.4 M
Worked example 2 (molality)
The same 5.85 g of NaCl is dissolved in 200 g of water.
Moles = 0.1 mol
Mass of solvent = 200 g = 0.200 kg
Molality = 0.1 / 0.200 = 0.5 mol/kg
Common mistakes:
1. Taking the mass of the solution (solute + solvent) instead of the solvent for molality.
2. Forgetting to convert mL to L or g to kg.
3. Using the volume of the solvent for molarity. It is always the final volume of the solution.
If a question gives density, you can convert between the two: mass of solution = density x volume, then subtract the mass of solute to get the mass of solvent.
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