Answer
(a) Neutral: pH = 7
(b) Basic: pH > 7
(c) Acidic: pH < 7
(d) Acidic: pH < 7
Work Step by Step
1. Identify if each ion is a strong or weak acid or base, or if it does not hydrolyze.
(a) $Na^+$ does not hydrolyze (Conjugate acid from strong base: NaOH)
$Br^-$ does not hydrolyze (Conjugate base from strong acid: HBr)
(b) $K^+$ does not hydrolyze (Conjugate acid from strong base: KOH)
$SO{_3}^{2-}$ hydrolyzes to produce $OH^-$ (Conjugate base from weak acid: $HSO{_3}^-$)
(c) $NH{_4}^{+}$ hydrolyzes to produce $H^+$ (Conjugate acid from weak base: $NH_3$)
$N{O_2}^-$ hydrolyzes to produce $OH^-$ (Conjugate base from weak acid: $HNO_2$)
(d) $Cr^{3+}$ hydrolyzes to produce $H^+$ (Small metal ion with high charge)
$NO_3^-$ does not hydrolyze (Conjugate base from strong acid: $HNO_3$)
2. If neither the anion nor the cation hydrolyze, the pH of the aqueous solution will be neutral (7). If the solution will produce only $H^+$, the aqueous solution will be acidic: pH < 7. If the solution only produces $OH^-$, the aqueous solution will be basic: pH > 7. If the solution produces both, we have to compare the strength of the electrolytes.
(a) Neutral: pH = 7
(b) Basic: pH > 7
(c) According to tables 15.3 and 15.4:
$NO_2^-$ has a Kb of $2.2 \times 10^{-11}$, and $NH_4^+$ has Ka = $5.6 \times 10^{-10}$
Therefore, $NH_4^+$, the acid, is stronger. The solution will be acidic: pH < 7
(d) Acidic: pH < 7
