Molarity and Formality are always the same
Both molarity and formality are expressed as number of moles of solute per litre of solution. So, do we need two distinct terms? Let's find out.
Molarity generally demonstrates the concentration of a particular chemical species in a solution. On the other hand, formality tells us the total concentration of the substance without any emphasis on its chemical form.
Say, if I ask you for the molarity of a solution when 0.1 mol of is dissolved in 1 L of water, what would be your most likely answer?
Is it 0.1 M ?
Before you come to that conclusion, think about what happens when an ionic compound dissolves in water. You probably know that it dissociates into its ions. So, gets dissociated into and ions when dissolved in water.
Molarity, as we said, expresses the concentration of a particular species in the solution - so, the solution will be 0.1M in and 0.2M in . And the molarity of will be zero, as there is no undissociated left.
However, as formality represents the total amount of , without any regards to its chemical form, it will still be 0.1 F.
So, if we are reading something like 0.1 M , we should know that it actually represents the concentration of the ions(, ) in the solution.
However, there are some substances which do not undergo dissociation in water. For these substances, the value of formality and molarity will remain the same. For example, a solution of glucose which has a concentration of 0.1 M will be 0.1 F as well.
To conclude, it can be said that in case of ionic compounds, such as, , , the term formality is more appropriate than molarity.