2
$\begingroup$

The following is a homework question:

Classify the aqueous solutions of the following ions as acidic, basic or neutral: $\ce{C6H5NH3+, Cl- , Fe^3+ , Mg^2+ , HSO4- , NO3- , NH4+ , Al^3+ , C5H5NH+, H2PO4-, CH3CO2-, Cu^2+, Na+, Ag+, ClO4-, PO4^3-, CN-, SH-, Ba^2+}$

I am looking for an approach to solve it.

My thought process so far:

  1. If any of the radical is highly reactive with $\ce{H+}$, then the solution will be basic as $\ce{OH-}$ is left. Similarly, if it reacts with $\ce{OH-}$, the solution will be acidic.

  2. However, this is where I am confused. Take $\ce{Fe^3+}$ for instance. It is known that it reacts to form $\ce{Fe(OH)3}$, which is a strong base. But, according to my logic above, the solution will be acidic.

Can someone help me with this and possibly give me hints at solving the rest?

$\endgroup$
  • $\begingroup$ $\ce{Fe(OH)3}$ is not a soluble compound. It is actually the hydrate of iron(iii) oxide hydroxide, which is formed when $\ce{Fe^3+}$ reacts with hydroxide anions. Since it pulls hydroxide out of solution, your intuition is correct. $\endgroup$ – Ben Norris Apr 15 '17 at 19:01
3
$\begingroup$

Your approach thus far is correct. $\ce{Fe^3+}$ is acidic; $\ce{Fe(OH)3}$ is not a strong base. Indeed, if $\ce{Fe(OH)3}$ were a strong base, it would dissociate and form $\ce{Fe^3+}$, but, as you've mentioned, the opposite process happens.

Your confusion may arise from the fact that $\ce{Fe^3+}$ is a Lewis acid, and produces protons in a "different" way than Arrhenius acids: \begin{align*} \ce{HCl &<=> H+ + Cl-}\\ \ce{Fe^3+ + H2O &<=> Fe(OH)^2+ + H+ }\end{align*}

Either way, your thought process (1.) will work for both cases.

$\endgroup$
  • $\begingroup$ Will Na+ solution also be acidic then? $\endgroup$ – Mahathi Vempati Apr 18 '17 at 17:33
  • 1
    $\begingroup$ @Tinkidinki, no. $\ce{Na+}$ does not react with $\ce{OH-}$. $\endgroup$ – a-cyclohexane-molecule Apr 19 '17 at 1:36

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.