pKa and predominant species

[Edward OKeefe IV]

I was given the following problem:

A monoprotic weak acid, HA , is ionized according to the reaction

HA(aq)+H2O(l)↽−−⇀A−(aq)+H3O+(aq)

pKa=4.10

where A− is the conjugate base to HA.

For this weak monoprotic acid, the predominant species present at pH 2.94 is:

charged
neutral
unknown

for which I assumed, as the pH is less than pKa, there would be a higher [H3O+] than at equilibrium; the solution, however, is "neutral" rather than "charged". Why is this so?

[905756606]

Hi! What I find helps me understand is that when pH<pKa, because the environment is already acidic, the equilibrium will shift to the left where the protonated from HA will dominate instead of producing more [H3O+]. Hope this helps!

[Briana Chavez 3F]

Hi! In general, if the pH is above the pKa, then the A- (charged) holds more power in the solution. On the other hand, if the pH is below the pKa, the HA (neutral) holds more power.

[Bruce Ruff 2B]

Because the pH is so low and is lower than the pKa, there is already a high concentration of H3O+, so the left side of the reaction (neutral side) would be favored. Creating even more H3O+ when there is already a lot of it would be unfavorable. I think this could have something to do with Le Chatelier's but I could be wrong.

[Neel Fulton 1D]

Hi! The way I think of it is that when the pH is less than the pKa, the reaction turning A- and H3O+ into HA and H2O is favored, as the system wants to increase the pH to reach the equilibrium concentration (making neutral HA the predominant species).

[Rio Gagnon 1G]

Since the pKa is greater than the pH, the acid is more present. In this case, the acid, HA, has no charge and therefore is neutral.