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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?

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  • $\begingroup$ $\ce{Fe(OH)3}$ 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
    Commented Apr 15, 2017 at 19:01

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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.

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  • $\begingroup$ Will Na+ solution also be acidic then? $\endgroup$
    – Mahathi Vempati
    Commented Apr 18, 2017 at 17:33
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    $\begingroup$ @Tinkidinki, no. $\ce{Na+}$ does not react with $\ce{OH-}$. $\endgroup$
    – a-cyclohexane-molecule
    Commented Apr 19, 2017 at 1:36

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