# Relative acidity of aqueous LiF, SnCl2, and YCl3

Which one of the following salts make the highest and lowest pH when dissolved in water? $$\ce{LiF, SnCl2, YCl3}$$

My approach was : considering the corresponding aqua acids, the order of acidity should be $\ce{Li(H2O)_n+ < Sn(H2O)_n^2+ < Y(H2O)_n^3+}$ and $\ce{F-}$ is less stable than $\ce{Cl-}$. So $\ce{LiF}$ is the weakest acid and $\ce{YCl3}$ the strongest.

But considering aqua acid looks weird (do they exist?) and I cannot be sure this of solution. I don't even know the right answer. Can anyone give some help?

• Hi, that's absolutely beautiful formatting, but please don't use MathJax in titles on chem.SE. And, regarding the question, I would expect the same trend that you have described. – orthocresol Sep 27 '16 at 11:06
• Yes, acidity of aquocomplexes is important here but it's more tricky than just charge. – Mithoron Sep 28 '16 at 17:43

Salt solutions are acidic because of hydrolysis.

LiF is obviously neutral, because it's a salt of very strong acid and very strong base.

Ironically, Wikipedia is wrong about Sn2+: it's much more acidic in water than Ca2+ or Zn2+, and more acidic than Y3+, because Sn2+ forms very stable and non-soluble hydroxo compounds and hence readily undergoes hydrolysis.

The answer is: LiF < YCl3 < SnCl2

Table data:

YCl3 has pH 4.5-5.8 (source),
SnCl2 has pH 1-2 (source 1, 2).

• I can’t start to explain how wrong it is to define deprotonation of aquacomplexes as hydrolysis (even if Wikipedia does so). Also, $\ce{LiF}$ is not the salt of ‘a very strong acid and a very strong base’. – Jan Oct 3 '16 at 18:59
• Well, if you can't start to explain your thoughts about hydrolysis, then at least explain why LiF is not a salt of strong acid and strong base. – sa7 Nov 9 '16 at 22:33
• Because neither is $\ce{HF}$ a strong acid nor is $\ce{Li-}$ — if that would even make sense — a strong base (You could argue about $\ce{LiOH}$, but that only semi made up the salt). On hydrolysis: It is the lysis (i.e. splitting) of a compound via the action of water. However, the acidic action in solution is not due to any lysis of any kind except for protolysis (displacement of a proton). The aqua ligands remain coordinated to the Lewis acid with one aqua ligand transforming itself (autolyticly!) into a hydroxido ligand. – Jan Nov 9 '16 at 22:40

A basic salt is formed when a weak acid reacts with a strong base. $$\ce{LiF}$$ is formed from strong base $$\ce{LiOH}$$ and weak acid $$\ce{HF}$$ to give a basic salt.

$$\ce{HF}$$ is not a strong acid.

• I attempted to restore intended formatting; please visit this page, this page and this one on how to format your future posts better with MathJax and Markdown. Also it seems like the answer is only partial; feel free to elaborate on two remaining compounds to make it complete. – andselisk Apr 14 at 19:52
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