Question 7.15: The Acid-Base Properties of Salts Predict whether an aqueous...
The Acid-Base Properties of Salts
Predict whether an aqueous solution of each of the following salts will be acidic, basic, or neutral.
a. NH_{4}C_{2} H_{3}O_{2} b. NH_{4}CN c. Al_{2} (SO_{4})_{3}
a. The ions in solution are NH_{4} ^{+} and C_{2} H_{3}O_{2} ^{-}. As we mentioned previously, K_{a} for NH_{4} ^{+} is 5.6 × 10 ^{-10} , and K_{b} for C_{2} H_{3}O_{2} ^{-} is 5.6 × 10 ^{-10} . Thus, since K_{a} for NH_{4} ^{+} is equal to K_{b} for C_{2} H_{3}O_{2} ^{-} , the solution will be neutral (pH = 7).
b. The solution will contain NH_{4} ^{+} and CN^{-} ions. The K_{a} value for NH_{4} ^{+} is 5.6 × 10 ^{-10}, and
K_{b} (for CN^{-} ) = \frac{K_{w} }{K_{a} (for HCN)} = 1.6 × 10 ^{-5}
Since K_{b} for CN^{-} is much larger than K_{a} for NH_{4} ^{+}, this solution will be basic.
c. The solution will contain Al(H_{2}O)_{6} ^{3+} and SO_{4} ^{2-} ions. The K_{a} value for Al(H_{2}O)_{6} ^{3+} is 1.4 × 10^{-5}, as given in Example 7.14. We must calculate K_{b} for SO_{4} ^{2-} . The H SO_{4} ^{-} ion is the conjugate acid of SO_{4} ^{2-} , and its K_{a} value is K_{a_2} for sulfuric acid, or 1.2 × 10 ^{-2}. Therefore,
K_{b} (for SO_{4} ^{2-} ) = \frac{K_{w} }{K_{a_2} (for sulfuric acid)}
= \frac{1.0 × 10 ^{-14}}{1.2 × 10 ^{-2}} = 8.3 × 10 ^{-13}
This solution will be acidic, since K_{a} for Al(H_{2}O)_{6} ^{3+} is much greater than K_{b} for SO_{4} ^{2-}.