As the pH number of an acid decreases, what happens to the strength of the acid?

1 Answer
zhirou
Apr 20, 2018

As #pH# increases, the acidity of the solution decreases.

Explanation:

First, let's begin with some definitions:

  • In the Bronsted-Lowry definition, acids are donors of #H^+# ions.
  • Strong acids are those that almost completely ionise in aqueous solutions to form these #H^+# ions.
  • #pH# is the negative #log# of the concentration of hydrogen cations (#[H^+]#) in an aqueous solution.

#pH = -log[H^+]#

Let's plug some numbers in for #[H^+]# and try to notice trends!:

#pH = -log(1.0xx10^(-2)) = 2.00#
#pH = -log(1.0xx10^(-3)) = 3.00#
#pH = -log(1.0xx10^(-4)) = 4.00#

We can see that, as #[H^+]# increases, the #pH# value decreases.

As the #pH# value increases, #[H^+]# decreases.

We defined a strong acid as one that almost completely ionises in aqueous solutions to form #H^+# ions. So, in strong acids, #[H^+]# will be high and #pH# will be low.

Therefore, as the #pH# of a solution increases, #[H^+]# would decrease and the acidity of the solution would also decrease as a result. :)

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