11
$\begingroup$

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?

| improve this question | |
$\endgroup$
  • 6
    $\begingroup$ 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. $\endgroup$ – orthocresol♦ Sep 27 '16 at 11:06
  • 4
    $\begingroup$ Yes, acidity of aquocomplexes is important here but it's more tricky than just charge. $\endgroup$ – Mithoron Sep 28 '16 at 17:43
1
$\begingroup$

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).

| improve this answer | |
$\endgroup$
  • $\begingroup$ 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’. $\endgroup$ – Jan Oct 3 '16 at 18:59
  • $\begingroup$ 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. $\endgroup$ – sa7 Nov 9 '16 at 22:33
  • $\begingroup$ 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. $\endgroup$ – Jan Nov 9 '16 at 22:40
0
$\begingroup$

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.

| improve this answer | |
$\endgroup$
  • 2
    $\begingroup$ 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. $\endgroup$ – andselisk♦ Apr 14 '19 at 19:52
  • 1
    $\begingroup$ Welcome to Chemistry.SE! To help you become acquainted with the site, I would just like to suggest if you haven't already, please take a minute to look over the help center and tour page to better understand our guidelines and question policies. You will find StackExchange (SE) to be a different kind of Q&A website from the mainstream. To learn more visit Meta.Chemistry, Meta.SE, or chat $\endgroup$ – A.K. Apr 14 '19 at 21:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.