Question 15.12: Determining Whether an Anion is Basic or pH-Neutral Classify...

(a) From Table 15.3, we can see that NO_{3}^{-} is the conjugate base of a strong acid (HNO_{3} ). NO_{3}^{-} is therefore, pH-neutral.
(b) From Table 15.5 (or from its absence in Table 15.3), we know that NO_{2}^{-} is the conjugate base of a weak acid (HNO_{2}). NO_{2}^{-} is therefore, a weak base.
(c) From Table 15.5 (or from its absence in Table 15.3), we know that C_{2}H_{3}O_{2}^{-} is the conjugate base of a weak acid (C_{2}H_{3}O_{2}). C_{2}H_{3}O_{2}^{-} is therefore, a weak base.

Table 15.3 Strong Acids
Hydrochloric acid (HCl)	Nitric acid (HNO_{3})
Hydrobromic acid (HBr)	Perchloric acid (HClO_{4})
Hydriodic acid (Hl)	Sulfuric acid (H_{2}SO_{4}) (diprotic)

TABLE 15.5 Acid Ionization Constants (K_{a}) for Some Monoprotic Weak Acids at 25 °C
Acid	Formula	Structural Formula	Ionization Reaction	K_{a}	pK*_{a}
Chlorous acid	HCIO_{2}	H—O—Cl=O	HClO_{2}(aq)+H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq)+ClO_{2}^{-}(aq)	1.1×10^{-2}	1.96
Nitrous acid	HNO_{2}	H—O—N=O	HNO_{2}(aq)+ H_{2}O(l) \xrightleftharpoons[]{} H_{3}O^{+}(aq) + NO_{2}^{–}(aq)	4.6×10^{-4}	3.34
Hydrouoric acid	HF	H—F	HF(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq) + F^{–}(aq)	3.5×10^{-4}	3.46
Formic acid	HCHO_{2}	H—O—\overset{O}{\underset{C}{\parallel } } —H	HCHO_{2}(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O+(aq) + CHO_{2}^{–}(aq)	1.8×10^{-4}	3.74
Benzoic acid	HC_{7}H_{5}O_{2}		HC_{7}H_{5}O_{2}(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq) + C_{7}H_{5}O_{2}^{–}(aq)	6.5×10^{-5}	4.19
Acetic acid	HC_{2}H_{3}O_{2}	H—O\overset{O}{\underset{C}{\parallel } } —CH_{3}	HC_{2}H_{3}O_{2}(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq) + C_{2}H_{3}O_{2}^{–}(aq)	1.8×10^{-5}	4.74
Hypochlorous acid	HCIO_{2}	H—O—CI	HClO(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq) + ClO^{–}(aq)	2.9×10^{-8}	7.54
Hydrocyanic acid	HCN	H—C≡N	HCN(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq)+CN^{–}(aq)	4.9×10^{-10}	9.31
Phenol	HC_{6}H_{5}O		HC_{6}H_{5}O(aq)+ H_{2}O(l)\xrightleftharpoons[]{}H_{3}O^{+}(aq)+C_{6}H_{5}O^{–}(aq)	1.3×10^{-10}	9.89
*pK_{a} = –logK_{a} (See the discussion at the end of Section 15.5)