Strong Acid vs Weak Acid: Comparing Strength of 2 Acids with different pH values

If there are two acids, acid A at pH1 and acid B at pH3, which acid is the stronger acid?

strong acid vs weak acid question - pH1 versus pH3

You might be thinking acid A is the answer.

Since it has a lower pH, therefore there are more H+ in solution A.

So A must be the stronger acid.

It's obvious right?

Well... not really.

The truth is we cannot determine the strength of an acid just by looking at its pH alone.

There are 2 instances where A is not the stronger acid.

1. Acid A is a weaker acid at higher concentration

Acid A can be a weak acid at a much higher concentration than Acid B which is a strong acid.

This means A can still dissociate to give more H+ in solution, hence its pH is lower.

strong acid vs weak acid scenario one - A is weak acid at higher concentration

2. Acids A and B are the same acid but acid A has a higher concentration

Now that we know that pH is dependent on concentration of the acid, we can also have the scenario that both acids A and B are the same strong acid, eg HCl.

If the concentration of acid A is higher, it will dissociate to give more H+ in solution, hence A will have a lower pH.

strong acid vs weak acid scenario 2 - same acid with different concentrations

Using pH alone to predict the strength of acids is a very common mistake among students for the topic of Ionic Equilibria.

Be sure to check this video out to clear this misconception once and for all!

Topic: Ionic Equilibria, Physical Chemistry, JC, H2, A Level Chemistry, Singapore

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