HW 2 - Neutral or charged species

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805758781
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HW 2 - Neutral or charged species

Postby 805758781 » Sun Jan 29, 2023 10:53 pm

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

HA(aq)+H2O(l)↽−−⇀A−(aq)+H3O+(aq)
pKa=4.00 where A− is the conjugate base to HA.

For this weak monoprotic acid, the predominant species present at pH 5.98 is charged, neutral or unknown?

I read the solution for this but I needed to clarify something.
"At pH values above the pKa, the charged, deprotonated species of A− will become more predominant in solution." is what the solution says. Now I understand this in the way that the pKa is a measure at equilibrium and the pH given is a measure at any other point in time for the reaction, so if pH is higher than pKa that means the amount of H+ in the solution is lesser than it should be at equilibrium so the system will move to increase amount of charged species. However, the questions asks "the predominant species present AT", not "what will become more predominant", therefore shouldn't the answer be neutral since to reach the equilibrium state we need more of the forward reaction to occur still?

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Re: HW 2 - Neutral or charged species

Postby Chem_Mod » Mon Jan 30, 2023 5:34 pm

The most important thing to know for these questions is that if pH < pKA then the species will stay protonated. Conversely, if the pH > pKA then the species will be predominantly deprotonated. Thus, in this example since pH > pKa, A- is the deprotonated species that will predominate, meaning that there will be a net charge.


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