Why do solutions of a strong acid, and a strong base, which contain equimolar #H_3O^+# and #HO^-#, when mixed give rise to a solution that is #pH# NEUTRAL?

2 Answers
Aug 2, 2015

Because 1:1 additions are in effect a neutralization reaction.


When common hydroxide salts (i.e. those of sodium or potassium) are added 1:1 to a strong acid, #HX#, a neutralization reaction occurs to give #NaX#, or #KX#.

i.e. #HX + MOH rarr MX + H_2O#.

Because #X^-# is the conjugate base of a strong acid, by definition it is very weakly basic and will not affect solution #pH#.

Aug 2, 2015

Simply put, because strong acids and strong bases dissociate completely in aqueous solution.


As you know, a salt is an ionic compound that is the product of a neutralization reaction, which occurs when an acid and a base react.

Strong acids and strong bases are characterized by the fact that they dissociate completely in aqueous solution.

#HA_((aq)) + H_2O_((l)) rightleftharpoons H_3O_((aq))^(+) + A_((aq))^(-)#

#B_((aq)) + H_2O_((l)) rightleftharpoons OH_((aq))^(-) + BH_((aq))^(+)#

This means that the equilibrium reaction that is established when they react with water will lie very far to the right. Essentially, this reaction can be seen as going to completion, meaning that the acid and the base will only exist as ions in solution.

#HA_((aq)) + H_2O_((l)) -> H_3O_((aq))^(+) + A_((aq))^(-)#

#B_((aq)) + H_2O_((l)) -> OH_((aq))^(-) + BH_((aq))^(+)#

Now, hydrolisys occurs when the cations and anions that result from the neutralization reaction react with water molecules, either with a hydroxide ion or with a hydronium ion.

For the conjugate base of a strong acid, this possible reaction with water would reform the strong acid, which is not possible.

#A_((aq))^(-) + H_3O_((aq))^(+) color(red)(cancel(color(black)(->))) HA_((aq)) + H_2O_((l))#

The strong acid will be completely dissociated in aqueous solution, so any attempt to reform it would simply not take place. Conjugate bvases of strong acids cannot accept protons from water.

The same is true for the conjugate acid of a strong base. It will not react with water to reform the strong base because of the one-way street of the aforementioned dissociation equilibrium

#BH_((aq))^(+) + OH_((aq))^(-) color(red)(cancel(color(black)(->))) B_((aq)) + H_2O_((l))#

Conjugate acids of strong bases cannot donate protons to water.

An important consequence of this fact is the neutral pH you get when a strong acid and a strong base react completely.