### Midterm Q3-C

Posted:

**Sun Mar 17, 2019 1:11 am**Can someone please explain the reasoning behind the answer for Q3C on the midterm please, Im thinking way too hard.

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=49&t=45205

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Posted: **Sun Mar 17, 2019 1:11 am**

Can someone please explain the reasoning behind the answer for Q3C on the midterm please, Im thinking way too hard.

Posted: **Sun Mar 17, 2019 1:33 am**

Partial Answer:

[HA] = 0

can't be true because they gave you [HA] in the problem already

[HA] = Ka

can't be true because of the relationship between pKa and Ka (pKa = -logKa)

[HA] = 0

can't be true because they gave you [HA] in the problem already

[HA] = Ka

can't be true because of the relationship between pKa and Ka (pKa = -logKa)

Posted: **Sun Mar 17, 2019 1:36 am**

If you write the equation out you get

HA +H2O -> A- +H3O+

you know that A- and H3O+ concentrations are equal to x and [HA] was 0.1-x so it can't be

[HA]=[H3O+] or [HA]=[A-]

so the answer is [HA] can't equal [H3O+]

HA +H2O -> A- +H3O+

you know that A- and H3O+ concentrations are equal to x and [HA] was 0.1-x so it can't be

[HA]=[H3O+] or [HA]=[A-]

so the answer is [HA] can't equal [H3O+]

Posted: **Sun Mar 17, 2019 1:51 am**

Thank you so much!

Posted: **Sun Mar 17, 2019 5:03 pm**

Given that pKa=10 , and that it is a weak acid, an amount x is ionized so there is an x amount of hydronium. However being that HA is a weak acid the amount ionized is very small compared to [HA] therefore it cannot equal [H30+].