Must Strong Acid be pH 1

In this JC2 webinar we want to discuss if all strong acids must be at pH 1.

Let's take a look at the following 3 solutions of strong acids at different concentrations.

must strong acid be pH 1 3 strong acids different concentration

If strong acids must always be at pH 1, we will be able to calculate that the pH of all the above HCl solutions must be 1.

1. 0.1 moldm-3 HCl

Since strong acids are fully dissociated,

[H+] = [HCl] = 0.1

pH = -lg[H+] = -lg(0.1) = 1

So the pH of 0.1 moldm-3 HCl will be 1.

2. 10-3 moldm-3 HCl

We can calculate pH of this strong acid in the exact same way.

[H+] = [HCl] = 10-3

pH = -lg[H+] = -lg(10-3) = 3

So the pH of 10-3 moldm-3 HCl will be 3.

3. 10-5 moldm-3 HCl

Finally we can find the pH of this last solution of HCl.

[H+] = [HCl] = 10-5

pH = -lg[H+] = -lg(10-5) = 5

So the pH of 10-5 moldm-3 HCl will be 5.

4. Conclusion

So we have shown by calculation that strong acids need not have pH 1.

It's a misconception that a solution of pH 1 must contain a strong acid, while a solution of pH 3 must contain a weak acid.

The pH of any acid is dependent on its concentration.

To determine if the acid is strong or weak, we have to compare acid concentration with H+ concentration, which is calculated from its pH.

must strong acid be pH 1 pH not reliable to determine strength

if [H+] = [acid], the acid is fully dissociated, hence strong.
if [H+] less than [acid], the acid is partially dissociated, hence weak.

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

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