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"#

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.)