Molarity and Formality

Both molarity and formality express concentration as moles of solute per liter of solution. There is, however, a subtle difference between molarity and formality. Molarity is the concentration of a particular chemical species. Formality, on the other hand, is a substance’s total concentration without regard to its specific chemical form. There is no difference between a compound’s molarity and formality if it dissolves without dissociating into ions. The formal concentration of a solution of glucose, for example, is the same as its molarity. (A solution that is 0.0259 M in glucose is 0.0259 F in glucose as well.)

For a compound that ionizes in solution, such as NaCl , molarity and formality are different. Dissolving 0.1 moles of CaCl2 in 1 L of water gives a solution containing 0.1 moles of Ca2+ and 0.2 moles of Cl−. The molarity of NaCl, therefore, is zero since there is essentially no undissociated NaCl. The solution, instead, is 0.1 M in Ca2+ and 0.2 M in Cl−. The formality of NaCl, however, is 0.1 F since it represents the total amount of NaCl in solution. The rigorous definition of molarity, for better or worse, is largely ignored in the current literature, as it is in this textbook. When we state that a solution is 0.1 M NaCl we understand it to consist of Na+ and Cl− ions. The unit of formality is used only when it provides a clearer description of solution chemistry.

Molarity is used so frequently that we use a symbolic notation to simplify its expression in equations and in writing. Square brackets around a species indicate that we are referring to that species’ molarity. Thus, [Na+]  is read as “the molarity of sodium ions.”