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

Md Ariful Islam

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Md Ariful Islam

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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 , 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 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.)

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