Question

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

Answer 1

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. :)