# Is the behavior of a dilute strong acid the same as the behavior of a weak acid?

My assumption is yes, as the $\mathrm{pH}$ levels of each will be similar and thus they have the same acidity. Is this correct or am I missing something?

The pH is the same, yes, but the weak acid has undissociated molecules "in reserve", so to speak, and thus can neutralize a lot more of any base.

Indeed, this concept of weak acids and their conjugate bases having molecules "in reserve", reacting only when called upon, is the principle behind acid-base buffers.

There are two main differences that can be observed, that Oscar's answer did not mention.

Firstly, when you add a strong base to the acid solution, the solution pH changes far more drastically when you had used a strong acid, simply because the amount of acid is very little to begin with.

Secondly, there is no buffer region for a strong acid solution. With a weak acid solution, at the rise in pH will slow down until a buffer region that lasts pretty much until nearly all the acid has been neutralized.

For instance a weak acid HA with disassociation constant $10^{-4.75}$ at concentration $0.1 \text{ mol} \text{ dm}^{-3}$ will have a pH of about $2.88$. A semi-strong acid with disassociation constant $10^{-2}$ will need to be at concentration about $0.0015 \text{ mol} \text{ dm}^{-3}$ to have the same pH. The pH of the solution as these two solutions are titrated against a strong base BOH can be easily proven to be the logarithm of the root of some cubic equation, which will have the following familiar shapes:

(This graph is when the added base is at concentration $0.2 \text{ mol} \text{ dm}^{-3}$. The blue curve is for the weak acid and the orange curve is for the semi-strong acid.)

• I didn't show the graph for a strong acid because it would be squashed too much to the left and there's really nothing to see. According to Wikipedia, what I called a semi-strong acid is still considered a weak acid, though on the strong side, but anyway it is strong enough for illustrative purposes. Jun 10 '16 at 8:13
• I am guessing the red line represents your "semi-strong" acid and the blue line represents the weak acid? It's a good idea to add it in imo Jun 10 '16 at 12:39
• @orthocresol: Done. I had forgotten about that! Jun 10 '16 at 13:07

The main difference between those two kinds of acid is the way that they dissociation occurs. A strong acid will rapidly dissociate in its ions with nearly a 100% of conversion p ex: HCl. Instead weak acids like acetic acid remain in equilibrium with its ions HAc <=> H+ + Ac- This means that in order to achieve a value of pH with a weak acid you need to consider that the acid will only partially increase the concentration of H+ ions so the final concentration will be given by the equilibrium constant (Ka) for this acid. Instead the pH given by a strong acid will be given simply by the concentration of the acid. So if you have HCl with a concentration of 10^-3 mol/L then the pH of that solution will be three. I hope this was helpful :)

• Just fyi, acetic acid is HOAc, AcH actually represents acetaldehyde Jun 10 '16 at 12:36