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In a solution of a salt formed by the reaction of a weak acid and a weak base, to predict the pH, we must know both the K a of the weak acid and the K b of the weak base. If K a > K b , the solution is acidic, and if K b > K a , the solution is basic.
(a) KBr
(b) NaHCO 3
(c) NH 4 Cl
(d) Na 2 HPO 4
(e) NH 4 F
(a) The K + cation and the Br − anion are both spectators, since they are the cation of a strong base (KOH) and the anion of a strong acid (HBr), respectively. The solution is neutral.
(b) The Na + cation is a spectator, and will not affect the pH of the solution; while the anion is amphiprotic, it could either behave as an acid or a base. The K a of is 5.6 10 −11 , so the K b of its conjugate base is
Since K b >> K a , the solution is basic.
(c) The ion is acidic and the Cl − ion is a spectator. The solution will be acidic.
(d) The Na + ion is a spectator, while the ion is amphiprotic, with a K a of 4.2 10 −13
so that the K b of its conjugate base is Because K b >> K a , 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 the K a and the K b of the two ions. K a of is 5.6 10 −10 , which seems very small, yet the K b of F − is 1.4 10 −11 , so the solution is acidic, since K a > K b .
(a) K 2 CO 3
(b) CaCl 2
(c) KH 2 PO 4
(d) (NH 4 ) 2 CO 3
(e) AlBr 3
(a) basic; (b) neutral; (c) basic; (d) basic; (e) acidic
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 simply as “Al 3+ ( aq )”, without explicitly noting the six water molecules that are the closest ones to the aluminum ion and just describing the ion as being solvated in water (hydrated). This is similar to the simplification of the formula of the hydronium ion, H 3 O + to H + . However, in this case, the hydrated aluminum ion is a weak acid ( [link] ) and donates a proton to a water molecule. Thus, the hydration becomes important and we may use formulas that show the extent of hydration:
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