Question

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"`

Answer 1

These are mostly straightforward, except for one.

• `"Al"("NO"_3)_3` is strongly acidic because `"Al"^(3+)` is a Lewis acid. It has an empty `3p_z` orbital that accepts electron density, making it an electron pair acceptor, a Lewis acid.
• `"C"_2"H"_5"NH"_3"NO"_3` is weakly acidic because the cation is the conjugate acid of the weak base ethylamine. `"NO"_3^(-)` doesn't contribute to the pH because it is the conjugate base of a strong acid (and is thus hardly a base).
• `"NaClO"` is weakly basic because `"ClO"^(-)` is the conjugate base of a weak acid, `"HClO"` (hypochlorous acid). `"Na"^(+)` forms a strong-base hydroxide, and so, it is hardly an acid.
• `"KCl"` is pH-neutral because `"Cl"^(-)` is the conjugate base of a strong acid (and is thus hardly a base). `"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 `"C"_2"H"_5"NH"_3"CN"`. Since `"C"_2"H"_5"NH"_3^(+)` and `"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 `"C"_2"H"_5"NH"_2` is about `4.3 xx 10^(-4)`, and the `K_a` of `"HCN"` is about `6.2 xx 10^(-10)`. At `25^@ "C"`,

`K_b("CN"^(-)) = K_w/K_a = 10^(-14)/(6.2 xx 10^(-10)) = ul(1.61 xx 10^(-5))`

`K_a("C"_2"H"_5"NH"_3^+) = (10^(-14))/(4.3 xx 10^(-4)) = ul(2.3 xx 10^(-11))`

Since the `K_a` of `"C"_2"H"_5"NH"_3^(+)` is much smaller than the `K_b` of `"CN"^(-)`, and both ions would be in solution, `"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 `"CN"^(-)` is still small.)