Molarity vs Molality: Formula and Definitions
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Both molarity and molality are measures of a chemical solution’s concentration. The primary difference between the two comes down to mass versus volume.
Contents
- Molality formula and units
- Molality equation
- Molarity formula and units
- Molarity equation
The molality describes the moles of a solute in relation to the mass of a solvent, while the molarity is concerned with the moles of a solute in relation to the volume of a solution.
Read on to learn more about molarity and molality, including their definitions, equations, and a comparison of the two terms.
Molality definition
Molality (m), or molal concentration, is the amount of a substance dissolved in a certain mass of solvent. It is defined as the moles of a solute per kilograms of a solvent. [Updated May 4, 2020].
Molality formula and units
The units of molality are m or mol/kg.
Molality equation
m = moles solute / kilograms solvent
Molarity definition
Molarity (M) is the amount of a substance in a certain volume of solution. Molarity is defined as the moles of a solute per liters of a solution. Molarity is also known as the molar concentration of a solution.
Molarity formula and units
The units of molarity are M or mol/L. A 1 M solution is said to be “one molar.”
Molarity equation
M = moles solute / liters solution
Molarity vs molality
An important distinction between molality and molarity is the difference between a solution and a solvent.
Molarity is the ratio of the moles of a solute to the total liters of a solution. The solution includes both the solute and the solvent.
Molality, on the other hand, is the ratio of the moles of a solute to the kilograms of a solvent. Note that the mass used in the denominator is that of only the solvent, not the solute and the solvent.
| | Molarity (M) | Molality (m) |
| Measure of | Concentration | Concentration |
| Definition | The moles of a solute per liters of a solution | The moles of a solute per kilograms of a solvent |
| Units | M | m |
| Equation | M = moles solute / liters solution | m = moles solute / kg solvent |
| Ratio of moles to: | Volume (in liters) | Mass (in kilograms) |
Molarity calculator
Find a handy calculator for determining the molarity of a solution here.
Molality calculator
A calculator for molality can be found here.
This article was updated May 4, 2020 to clarify the definition of molality.
