What is pH of a salt solution?

1 Answer
anor277
Aug 16, 2017

The #pH# of the salt of a strong acid and a strong base is usually near as dammit to #pH=7# in aqueous solution.

Explanation:

And thus for strong acids mixed with 1 equiv of strong base we get....

#"KOH(aq)" +" HCl(aq)" rarr "KCl(aq)" + "H"_2"O(l)"#

#"NaOH(aq) + HClO"_4"(aq)" rarr "NaClO"_4(aq) + "H"_2"O(l)"#

Because neither chloride nor perchlorate is particularly basic (they are the conjugate bases of strong acids, and thus they do not compete strongly for the protium ion), the #pH# of the solution is very close to #7#.

On the other hand, when 1 equiv acetic acid, a WEAK acid, is treated with 1 equiv of sodium hydroxide, the resultant solution is stoichiometric in #"H"_3"CCO"_2^(-)"Na"^(+)#. And this salt causes some hydrolysis, i.e.

#"H"_3"CCO"_2^(-)"Na"^(+)+"H"_2"O"(l) rightleftharpoons "H"_3"CCO"_2"H(aq)" +"NaOH(aq)"#

And thus at equivalence, the #pH# of the solution will offset upwards from #pH=7# reflecting the equilibrium quantity of sodium hydroxide in solution.

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