What is Molarity and Molality?
Molarity (M) and molality (m) are two key ways to express the concentration of a solute in a solution. Both measure how much solute is dissolved, but use different reference quantities—making each useful in different contexts.
Molarity (M) = moles of solute ÷ liters of solution. Molality (m) = moles of solute ÷ kilograms of solvent. Molarity changes with temperature; molality is temperature-independent.
- •Moles ÷ liters of solution
- •Temperature-dependent
- •Most common in labs
- •Perfect for stoichiometry
- •Moles ÷ kilograms of solvent
- •Temperature-independent
- •Used for colligative properties
- •Better for precise work
Try it: interactive calculator
Step-by-step worked examples
How many moles of NaCl are dissolved in 2 L of a 1.5 M solution?
n = M × V n = 1.5 mol/L × 2 L n = 3 mol NaCl
What is the molality of 4 mol of glucose dissolved in 1.5 kg of water?
m = n / kg of solvent m = 4 mol / 1.5 kg m = 2.67 mol/kg
Calculate molarity: 0.8 mol of KCl in 400 mL of solution.
Convert mL to L: 400 mL = 0.4 L M = 0.8 mol / 0.4 L M = 2 mol/L
Flashcards
Quick quiz
Q1.What is the molarity of a solution with 0.5 mol solute in 500 mL?
Q2.Molarity depends on which variable?
Q3.A 3 mol/kg molality solution means…
Q4.Which unit is independent of temperature?
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Common mistakes
Using liters of solvent instead of liters of solution in molarity. — Correct: Molarity uses the total volume of the final solution, not just the solvent volume.
Treating molarity and molality as the same thing. — Correct: Molarity uses volume (M), molality uses solvent mass (m). They differ by definition.
Assuming molarity stays constant when temperature changes. — Correct: As temperature changes, solution volume changes, so molarity changes; molality remains constant.
Dividing moles by the mass of the whole solution for molality. — Correct: Molality uses only the mass of the solvent, not the total solution mass.
FAQ
What is the formula for molarity?
Molarity = moles of solute ÷ liters of solution (M = n/V).
What is the formula for molality?
Molality = moles of solute ÷ kilograms of solvent (m = n/kg).
When should I use molality instead of molarity?
Use molality when studying colligative properties (freezing point, boiling point), because it's temperature-independent.
Why does molarity change with temperature but molality doesn't?
Because volume changes with temperature, but mass does not.