Which of these is the strongest acid? #"HCl"#, #"H"_3"PO"_4#, #"CH"_3"COOH"#

2 Answers
Mar 4, 2017

#"HCl"# is the only strong acid in this selection of acids.

The highest #["H"^(+)]# comes from the acid, #"HA"#, that dissociates into #"H"^(+)# and #"A"^(-)# (its conjugate base) the most:

#"HA"(aq) + "H"_2"O"(l) rightleftharpoons "H"_3"O"^(+)(aq) + "A"^(-)(aq)#

(note that #"H"_3"O"^(+)# and #"H"^(+)# are different representations for equivalent species.)

By definition, a strong acid dissociates pretty much completely, so it must give the most #"H"^(+)# into solution.

This can be quantified by their acid dissociation constants, #K_a#:

#"HCl"#: #K_a ~~ 10^7#

#"H"_3"PO"_4#: #K_(a1) = 6.9 xx 10^(-3)#

#"CH"_3"COOH"#: #K_a = 1.76 xx 10^(-5)#

Based on the values of the #K_a#'s, which do you think is the weakest acid (considering only the first proton)?

Mar 4, 2017

Answer:

Because #HCl# is the strongest acid...........

Explanation:

When we talk of the strength of an acid, we refer to the following equilibrium reaction in water:

#HX(aq) rightleftharpoons H^(+) + X^-#

The further the equilibrium lies to the right hand side as we face the page, the STRONGER is the acid. (Sometimes we represent #H^+# as the hydronium ion, #H_3O^+#, i.e. protonated water:

#HX(aq) + H_2O(l) rightleftharpoons H_3O^+ + X^-#)

For the LOWER hydrogen halides, #HCl#, #HBr#, #HI#, (but not for #HF)#, the given equilibrium lies almost quantitatively to the right. Thus #1*mol*L^-1# solutions of #HCl#, #HBr#, are almost quantitative in #H^+#, i.e. #H^(+)=1.0*mol*L^-1#

For acetic acid and phosphoric acid, the equilibrium would not go to completion:

#H_3"CCO"_2H(aq) rightleftharpoons H_3"CCO"_2^(-) +H^+#

Mind you phosphoric acid has a lower #pK_(a1)# than acetic acid.