The key difference between molarity and molality is that molarity is the number of moles of solute present in 1 litre of solution whereas molality is the number of moles of solute present in 1kg of solvent.
Molarity and Molality are measures of concentrations of solutions. The concentration of a solution gives the amount of solutes dissolved in a unit amount of solution. The two measures are different from each other according to the parameter that we use to measure the unit amount of solution; molarity considers the volume of the solution while molality considers the mass of the solvent. However, in both cases, we measure the number of solutes in moles.
CONTENTS
1. Overview and Key Difference
2. What is Molarity
3. What is Molality
4. Side by Side Comparison – Molarity vs Molality in Tabular Form
5. Summary
What is Molarity?
Molarity is the concentration of a solution given by the number of solutes present in a litre of solution. Therefore, we measure the concentration of a solution by means of volume when determining the molarity. The unit of measurement is mol/L.
Figure 01: Different Solutions have different Concentrations depending on the Volume of the Solution
Furthermore, this parameter depends upon the temperature of the solution because the volume of a solution can vary with the temperature, i.e. almost all the times the volume of liquids expands with increasing temperature. For instance, a solution containing two moles of solute dissolved in a litre of solution has 2.0 mol/L concentration. We can denote the molarity by “M”.
What is Molality?
Molality is the concentration of a solution given by the amount of solutes present in one kilogram of solvent. Therefore, we measure the concentration of a solution by means of the mass of solvent when determining the molality of the solution. Hence, the unit of measurement is mol/kg.
Figure 02: Determination of Molality of a Solution
Moreover, this parameter does not depend on the temperature of the solution because the mass does not change with temperature. For example, if a solution contains 2 moles of solutes dissolved in one kilogram of solvent, then the concentration is 2.0 mol/kg. We usually denote this term as “m”.
What is the Difference Between Molarity and Molality?
Molarity is the concentration of a solution given by the amount of solutes present in a litre of solution whereas molality is the concentration of a solution given by the amount of solutes present in a kilogram of solvent. Therefore, this is the key difference between molarity and molality. Furthermore, an important difference between molarity and molality is that the molarity depends upon the temperature of a solution while the molality is independent of the temperature. This is mainly because the volume can expand with increasing temperature whereas the mass remains constant upon temperature changes.
The below infographic shows the difference between molarity and molality in tabular form.
Summary – Molarity vs Molality
Molarity and Molality are measures of concentrations of solutions. The key difference between molarity and molality is that the molarity is the number of moles of solute present in 1 litre of solution whereas the molality is the number of moles of solute present in 1kg of solvent.
Reference:
1. Mott, Vallerie. “Introduction to Chemistry.” Lumen. Available here
2. “Molar Concentration.” Wikipedia, Wikimedia Foundation, 17 Oct. 2018. Available here
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