What is the Difference Between Formality and Molarity?

Formality and molarity are both measures of concentration in a solution, but they differ in how they account for the dissociation of solutes. Here are the key differences between formality and molarity:

1. Definition: Molarity is the concentration of a particular chemical species, expressed as the ratio of moles of the substance to the volume of the solution it forms. Formality, on the other hand, is a substance's total concentration without regard to its specific chemical form.
2. Dissociation: Molarity takes into account the dissociation of solute into ions, while formality does not depend on the dissociation of the solute after dissolving.
3. Applicability: Molarity is used for molecular substances, while formality is used for ionic substances.
4. Unit: The unit of molarity is mol/m³, while the unit of formality is form/m³ (where "form" is the number of gram formal masses present).
5. Differences in Values: For a compound that ionizes in solution, such as CaCl2, molarity and formality are different. For example, when 0.1 moles of CaCl2 is dissolved in 1 L of water, the solution contains 0.1 moles of Ca²⁺ and 0.2 moles of Cl⁻. The molarity of CaCl2 is zero since there is no undissociated CaCl2 in solution.

In summary, molarity and formality are both measures of concentration in a solution, but they differ in their accounting for the dissociation of solutes. Molarity is used for molecular substances and takes into account the dissociation of solutes into ions, while formality is used for ionic substances and does not depend on the dissociation of the solute after dissolving.

Comparative Table: Formality vs Molarity

Formality and molarity are both measures of concentration in chemistry, but they differ in their application and definition. Here is a table summarizing the differences between the two:

In summary, formality is used for ionic substances, while molarity is used for molecular substances. Both measures express concentration as moles of solute per liter of solution, but they differ in their application and definition.