Today, I came across a question: "State the differences, in not less than 5 points, between Formality and Molarity".

Using my knowledge, I wrote down 2 points, and stretched it to 3 somehow, but that's all I could do... Also, in the process, I got confused between the two, because in pH calculations, even when Ionic solids and acids are dissolved in water, their concentration is expressed in terms of molarity, and not formality.

Please note that I am not asking this for the sake of my homework, but am asking for a better scientific knowledge of Formality, and how it differs from Molarity.

Here are the 3 points I wrote:

  1. Molarity is the concentration of a molecular substance, expressed in terms of the ratio of moles of substance present to the volume of the solution it forms. Formality, on the other hand, is the concentration of an Ionic substance, that exists in the form of an Ionic Crystal Lattice, in terms of the ratio the gram formal masses present to the volume of the solution it forms.

  2. Molarity is defined for molecular substances, whereas Formality is defined for Ionic substances.

  3. The unit of molarity is $\pu{mol m-3}$, while that of formality is $\mathrm{form}~\pu{m-3}$ (where $\mathrm{form}$ is the number of gram formula mass units).

Can someone please explain the actual differences, and help with expanding the answer for more accuracy?

  • 2
    $\begingroup$ I have never come across formality before. $\endgroup$ – Jan Sep 17 '17 at 11:52
  • $\begingroup$ @Jan Really? I thought it was a standard form of concentration, used alongside Molarity, Molality, Normality, etc. No wonder there's no answers yet... $\endgroup$ – AbhigyanC Sep 17 '17 at 11:56
  • 3
    $\begingroup$ Molarity is what I always come across. I have come across normality in a few instances (the diluted acid stock solutions in Munich were all 2 N, meaning that sulphuric acid was half-concentrated with respect to hydrochloric and nitric acid). A chemistry high school course once briefly touched molality; I have since forgotten its definition. That’s how standard they are ;) $\endgroup$ – Jan Sep 17 '17 at 12:06

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