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Chapter 14 | Acid-Base Equilibria 765
Since Kb >> Ka, the solution is basic.
(c) The ion is acidic and the Cl− ion is a spectator. The solution will be acidic.
(d) The Na+ cation is a spectator, and will not affect the pH of the solution, while the anion is amphiprotic. The Ka of is 4.2 10−13,
and its Kb is Because Kb >> Ka, the solution is basic.
(e) The ion is listed as being acidic, and the F− ion is listed as a base, so we must directly compare theKaandtheKbofthetwoions.Kaof is5.6 10−10,whichseemsverysmall,yettheKbofF−is 1.4 10−11, so the solution is acidic, since Ka > Kb.
Check Your Learning
Determine whether aqueous solutions of the following salts are acidic, basic, or neutral: (a) K2CO3
(b) CaCl2
(c) KH2PO4
(d) (NH4)2CO3 (e) AlBr3
Answer:
(a) basic; (b) neutral; (c) acidic; (d) basic; (e) acidic
The Ionization of Hydrated Metal Ions
If we measure the pH of the solutions of a variety of metal ions we will find that these ions act as weak acids when in solution. The aluminum ion is an example. When aluminum nitrate dissolves in water, the aluminum ion reacts with water to give a hydrated aluminum ion, dissolved in bulk water. What this means is that the
aluminum ion has the strongest interactions with the six closest water molecules (the so-called first solvation shell), even though it does interact with the other water molecules surrounding this cluster as well:
We frequently see the formula of this ion written simply as “Al3+(aq)”, without explicitly noting that six water molecules are covalently bonded to the aluminum ion. This is similar to the simplification of the formula of the hydronium ion, H3O+ to H+. In this case, a bonded water molecule acts as a weak acid (Figure 14.16) and donates a proton to a water molecule.
The conjugate base produced by this process contains five other bonded water molecules capable of acting as acids,
and so the sequential or step-wise transfer of protons is possible as depicted in the equations below:
This is an example of a polyprotic acid, the topic of discussion in a later section of this chapter.
