# Identify the following compounds as strongly acidic, weakly acidic, weakly basic, strongly basic, or pH-neutral in water at 25^@ "C"? "Al"("NO"_3)_3, "C"_2"H"_5"NH"_3"NO"_3, "NaClO", "KCl", "C"_2"H"_5"NH"_3"CN"

Nov 3, 2017

These are mostly straightforward, except for one.

• "Al"("NO"_3)_3 is strongly acidic because ${\text{Al}}^{3 +}$ is a Lewis acid. It has an empty $3 {p}_{z}$ orbital that accepts electron density, making it an electron pair acceptor, a Lewis acid.
• ${\text{C"_2"H"_5"NH"_3"NO}}_{3}$ is weakly acidic because the cation is the conjugate acid of the weak base ethylamine. ${\text{NO}}_{3}^{-}$ doesn't contribute to the pH because it is the conjugate base of a strong acid (and is thus hardly a base).
• $\text{NaClO}$ is weakly basic because ${\text{ClO}}^{-}$ is the conjugate base of a weak acid, $\text{HClO}$ (hypochlorous acid). ${\text{Na}}^{+}$ forms a strong-base hydroxide, and so, it is hardly an acid.
• $\text{KCl}$ is pH-neutral because ${\text{Cl}}^{-}$ is the conjugate base of a strong acid (and is thus hardly a base). ${\text{K}}^{+}$ forms a strong-base hydroxide, and so, it is hardly an acid. Thus, the result is pH-neutral.

The only thing I would question is $\text{C"_2"H"_5"NH"_3"CN}$. Since ${\text{C"_2"H"_5"NH}}_{3}^{+}$ and ${\text{CN}}^{-}$ are a weak acid and a weak base, respectively, I would have to actually check their strengths and make a judgment from there.

The ${K}_{b}$ of ${\text{C"_2"H"_5"NH}}_{2}$ is about $4.3 \times {10}^{- 4}$, and the ${K}_{a}$ of $\text{HCN}$ is about $6.2 \times {10}^{- 10}$. At ${25}^{\circ} \text{C}$,

${K}_{b} \left({\text{CN}}^{-}\right) = {K}_{w} / {K}_{a} = {10}^{- 14} / \left(6.2 \times {10}^{- 10}\right) = \underline{1.61 \times {10}^{- 5}}$

${K}_{a} \left({\text{C"_2"H"_5"NH}}_{3}^{+}\right) = \frac{{10}^{- 14}}{4.3 \times {10}^{- 4}} = \underline{2.3 \times {10}^{- 11}}$

Since the ${K}_{a}$ of ${\text{C"_2"H"_5"NH}}_{3}^{+}$ is much smaller than the ${K}_{b}$ of ${\text{CN}}^{-}$, and both ions would be in solution, ${\text{CN}}^{-}$ dominates as the base by a factor of about $1000000$, and this salt is weakly basic.

(It's not strongly basic because the ${K}_{b}$ of ${\text{CN}}^{-}$ is still small.)