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Author: Subject: Understanding Formality
RU_KLO
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[*] posted on 28-1-2025 at 14:39
Understanding Formality


first the question:

I need 3F (formal) H2SO4.

This is 3M H2SO4 or 6M H2SO4 or 1,5M H2SO4?

I undestand molarity, normality, but formality no.

"Molarity and Formality
Both molarity and formality express concentration as moles of solute per liter of solution; however, there is a subtle difference between them. 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.

For a compound that ionizes in solution, such as CaCl2, molarity and formality are different. When we dissolve 0.1 moles of CaCl2 in 1 L of water, the solution contains 0.1 moles of Ca2+ and 0.2 moles of Cl–. The molarity of CaCl2, therefore, is zero since there is no undissociated CaCl2 in solution; instead, the solution is 0.1 M in Ca2+ and 0.2 M in Cl–. The formality of CaCl2, however, is 0.1 F since it represents the total amount of CaCl2 in solution. This more rigorous definition of molarity, for better or worse, largely is ignored in the current literature, as it is in this textbook. When we state that a solution is 0.1 M CaCl2 we understand it to consist of Ca2+ and Cl– ions. We will reserve the unit of formality to situations where 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, [Ca2+] is read as “the molarity of calcium ions.”

https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/02%3A_Basic_Tools_of_Analytical_Chemistry/2.02%3A_Conce ntration#:~:text=Molarity%20and%20Formality,-Both%20molarity%20and&text=Molarity%20is%20the%20concentration%20of,dissolves%20without%20dissociatin g%20into%20ions.

thanks.




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[*] posted on 28-1-2025 at 17:28


3 M H2SO4 is 3 F H2SO4. It's not precise to say 3 M H2SO4, because there isn't actually any undissociated H2SO4 in solution, so they use 3 F to say that we don't care if it's really still H2SO4 or a bunch of ions at this point.



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[*] posted on 28-1-2025 at 22:00


from the explanation provided I think that
1M h2so4 is equivalent to a 3F solution of ions.
EDIT: on being corrected (below) and after re-reading ... OOPS! I am wrong. Sorry.


[Edited on 29-1-2025 by Sulaiman]




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[*] posted on 29-1-2025 at 00:11


No, if I read the definition in the original post, then 1M H2SO4 will be close to 1F H2SO4.

Formaility can be a useful concept. Take H3PO4 for instance. this is a moderately strong acid in the first proton, and a weak acid in the second proton.

If I dissolve 1 mole of H3PO4 and top this up to 1 liter of water, then all phosphorus (in +5 oxidation state, phosphate) is distributed over H3PO4, H2PO4(-), HPO4(2-) and maybe even traces of PO4(3-). The sum of all concentrations of all phosphorus-containing species is 1 F. So you can say that the formal phosphate concentration is 1 F. The concentration of the individual ions is not precisely known (can be computed if you have precise kPa figures for the different stages of deprotonation).




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