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Molarity vs Molality: Formula and Definitions
Article

Molarity vs Molality: Formula and Definitions

Molarity vs Molality: Formula and Definitions
Article

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.

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.

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