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### Acid and [H3O] Midterm Q

Posted: Tue Feb 11, 2020 10:01 am
I found this question from a study guide from a few years ago and was hoping someone could explain the answer (in bold).

For a 0.10 M solution of a weak acid, HA, with pKa = 10, which of the following is true?

[HA] ≅ 0
[HA] = Ka
[HA] = [A−]
[HA] ≠ [H3O+]
[HA] = [H3O+

### Re: Acid and [H3O] Midterm Q

Posted: Tue Feb 11, 2020 1:51 pm
use an ice table with [HA] = 0.10 M as HA's initial concentration

### Re: Acid and [H3O] Midterm Q

Posted: Wed Feb 12, 2020 12:31 pm
Because they say its a weak acid, you know that the HA is never going to fully dissociate into H30+, so HA doesnt equal H30+. You can calculate HA and it does not equal approximately 0. You have the pka and to find ka, you just do 10^-pka, giving you ka. Then you set that equal to the concentration of product over reactants, giving you the Ka equal to x^2 over the M they gave you, .1. Then you just solve for x by multiplying denominator across and then square rooting both sides, giving you x and plug that in for H30+ and you have the concentration for it.

### Re: Acid and [H3O] Midterm Q

Posted: Wed Feb 12, 2020 12:40 pm
Since HA is a weak acid, it will not completely dissociate into H3O+ ions. This means that the initial concentration of HA will not be the same as the final concentration of H3O+ ions.