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Hydrochloric acid is a strong acid and acetic acid is a weak acid. Which of the following correctly portrays aqueous solutions of the two acids?

a. HCl(aq) and HC2H3O2(aq)

b. H+(aq) + Cl-(aq) and HC2H3O2(aq)

c. HCl(aq) and H+(aq) + C2H3O2-(aq)

d. H+(aq) + Cl-(aq) and H+(aq) + C2H3O2-(aq)

e. none of the above


I know HCl will split up into its ions since it is a strong acid.  What is giving me trouble is that I am aware that weak acids partially dissociate in aqueous solutions, but I am unsure if I would represent it by showing the hydrogen ion and acetate ion split up since it doesn't fully dissociate.  Please explain which answer is correct so I can understand!  Thanks.



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2 Answers

It's B. The intent of the question is to get you to distinguish between the behavior of strong and weak acids in water. Strong acids dissociate completely, whereas weak acids dissociate only partially. We think of the *majority* of the weak acid as remaining intact, which will be true at most normally used concentrations -- once a few molecules have dissociated, the solution pH will be below the pKa of HAc. However, if the acetic acid is so dilute that even 100% dissociation would not drive the pH to below the pKa, then obviously the conditions are favorable for greater than 50% dissociation of the acetic acid molecules, since exactly half of the molecules would be dissociated if the resultant pH matches the pKa = 4.75. You might think this occurs when [HAc]initial = 2 * (10^-4.75) molar, but normally there's some dissolved CO2 lowering the pH on its own, so that it won't take *quite* so much acetic acid to get to pH = 4.75.

I second the remark above about "strong acid" and "weak acid" as not telling you about risk. H3PO4, HAc, HF, HCN and H2S (all weak acids!) are all examples of extremely hazardous agents for human exposure, in part because they do not trigger the immediate pain of H+ exposure.

On the other hand (pun intended), a spill of a base onto skin anesthetizes it so that extreme damage can occur painlessly. Your only clue might be that your skin feels slippery. If you are ever working with a possible base and you notice that, immediately rinse your skin thoroughly!

HCl  +  HAc (Acetic Acid) <—> H++  Cl -  +  H+  +  Ac-.

The strong dissociation of HCl drives the H+  Ac-  to the left of the equilibrium.  Think of the HCl pushing to the right, and the acetic acid ions pushing to the left both at the same time.

B, is the correct answer. 

Acetic acid is relatively weak.  Don't be deceived.  Glacial acetic acid is nasty, too.