Can pH be Negative?

In this JC2 webinar we want to discuss if pH of an acidic solution can be negative.

Let's take a look at this question.

can pH be negative question

Calculating the pH of a strong acid is pretty straightforward.

We know strong acids are fully dissociated so the concentration of H+ and HCl are equal.

can pH be negative strong acid full dissociation

We can then calculate the pH from the H+ concentration to get a negative value.

can pH be negative calculate pH

This result might confuse some of us, as we usually think the lowest possible pH is either 1 or zero.

The reason why we have that concept is due to the pH scale that we are very familiar with.

can pH be negative pH scale

Notice the pH scale gives us the following impressions which are not always true!

- pH of neutral water is always 7
- the strongest possible acid has a minimum pH of 0 (or 1 depending on the textbook that you were using)
- the strongest possible base has a maximum pH of 14

Let's calculate the H+ concentration that corresponds to pH 0.

can pH be negative conc of H plus for pH 0

The concentration of H+ at pH 0 is only 1.0 mol dm-3.

It's definitely possible to increase the concentration of H+ to beyond 1.0 mol dm-3, and pH will become negative.

Similarly, we can calculate the OH- concentration that corresponds to pH 14.

can pH be negative conc of OH minus for pH 14

Again it's possible to increase the concentration of OH- to more than 1.0 mol dm-3 and pH will be greater than 14.

It's important to note that many ideas from the pH scale that we take for granted are actually misconceptions.

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

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