# G chem.

#### Danny289

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If the pH of a 0.10 M weak solution is 2.70, what is the concentration of the conjugate base?

A. 0.10 M
B. 2.0 x 10^-2 M
C. 2.0 x 10^-3 M
D. 2.0 x 10^-4 M

what you think? and why

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#### osimsDDS

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hmm i think how you do this is you take the pH= 2.7 = about 2 x10^-3 (H+ concentration aka Ka) and then subtract that from .1M and you should get something x10^-2 so i would go with B

#### osimsDDS

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are we wrong Danny?? #### harrygt

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assuming this is a weak monoprotic acid, the concentration of H+ in the solution is going to be equal to the concentration of the conjugate base. Therefore, it should be 10^-2.7 which is close to 2 * 10^-3.
I would pick C.

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#### osimsDDS

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assuming this is a weak monoprotic acid, the concentration of H+ in the solution is going to be equal to the concentration of the conjugate base. Therefore, it should be 10^-2.7 which is close to 2 * 10^-3.
I would pick B.

you mean C harry, 2x10^-3 is answer choice C...

I actually agree with you, because the dissociation assuming its a monoprotic acid like you said will make up H+ and the conjugate base, therefore the H+ concentration should equal the conjugate base concentration...

#### Danny289

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you all are wrong! pure conceptual! rethink you are in right track but.....

#### PooyaH

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B sounds reasonable to me! for the exact same reason Osims said!

HCl -> H+ + Cl-

So if [H+] is around 2 x 10^-3, then [Cl-] = .1M - (2x10^-3)

#### harrygt

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you mean C harry, 2x10^-3 is answer choice C...

I actually agree with you, because the dissociation assuming its a monoprotic acid like you said will make up H+ and the conjugate base, therefore the H+ concentration should equal the conjugate base concentration...

Yeah, I meant choice C. Sorry about that. It was just a typo. Danny, you mean the answer is not C?

#### Danny289

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assuming this is a weak monoprotic acid, the concentration of H+ in the solution is going to be equal to the concentration of the conjugate base. Therefore, it should be 10^-2.7 which is close to 2 * 10^-3.
I would pick C.
you are rightttttttttttttttttttt> king of chem beautiful resoning

#### harrygt

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B sounds reasonable to me! for the exact same reason Osims said!

HCl -> H+ + Cl-

So if [H+] is around 2 x 10^-3, then [Cl-] = .1M - (2x10^-3)

Pooya, what you are calculating is the concentration of HCl left.

#### HoangDDS

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you mean C harry, 2x10^-3 is answer choice C...

I actually agree with you, because the dissociation assuming its a monoprotic acid like you said will make up H+ and the conjugate base, therefore the H+ concentration should equal the conjugate base concentration...

if the PH of the weak acid is 2.7, then the [H+] = 2 x 10^-3M , but I dont understand why that would equal the conjugate base?

#### osimsDDS

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wait so what is the correct answer???

#### PooyaH

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Pooya, what you are calculating is the concentration of HCl left.

Whoops, you're right!

#### harrygt

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you are rightttttttttttttttttttt> king of chem

Lol, what do you mean I'm very close? You mean it is not C yet? I meant 2 * 10^-3 at the begining, But I though it was choice B.

#### Danny289

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Lol, what do you mean I'm very close? You mean it is not C yet? I meant 2 * 10^-3 at the begining, But I though it was choice B.

I edit my post the answer is C ecatly for the reason you explained no need for calcalution!!!

#### harrygt

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10+ Year Member  OH ok. I'm glad we are finally done with this  Bro, once you said I'm very close, my heart stopped.

#### HoangDDS

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if the PH of the weak acid is 2.7, then the [H+] = 2 x 10^-3M , but I dont understand why that would equal the conjugate base?

AHHH... nm. The conjugate base is H30+.

for a weak acid
HA + H20 <---> H30+ + A-

so given pH= 2.7, then H3O+ = 2x10^-3M

#### harrygt

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if the PH of the weak acid is 2.7, then the [H+] = 2 x 10^-3M , but I dont understand why that would equal the conjugate base?

if x amount of HA dissociates , you will have x amount of H+ and x amount of A- [the conjugate base]. Therefore, they will be equal.

#### osimsDDS

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So basically its like this:

HAC ----> AC- + H+

pH= [H+] = 2x10^-3 which would also be the same concentration for AC-

#### harrygt

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AHHH... nm. The conjugate base is H30+.

for a weak acid
HA + H20 <---> H30+ + A-

so given pH= 2.7, then H3O+ = 2x10^-3M

nope! H3O+ is the same as H+. You can simplify and right it as HA <---> H+ + A-
A- is the conjugate base.

#### osimsDDS

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but if it was a diprotic acid it would still be like this correct:

H2AC ----> HAC- + H+

it would still be the same concentration tho for H+ and HAC-

#### Danny289

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So basically its like this:

HAC ----> AC- + H+

pH= [H+] = 2x10^-3 which would also be the same concentration for AC-

yes bro, you got it. you didn't realy need to calculate. just should know the simple concept.

#### HoangDDS

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yah that .1M weak solution threw me off. If only if they asked what's the concentration of H+. Bam C.

#### harrygt

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but if it was a diprotic acid it would still be like this correct:

H2AC ----> HAC- + H+

it would still be the same concentration tho for H+ and HAC-

Yeah, but if you consider this H2AC ---> 2H+ + AC - then it would not. Anyway, that question should have been more specific.

#### Glycogen

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if x amount of HA dissociates , you will have x amount of H+ and x amount of A- [the conjugate base]. Therefore, they will be equal.

#### harrygt

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We could have used Henderson-... if we had the pka of the acid.

PH - pka = log A-/HA

This is really simple. if 1 mole of 10 moles of HA dissociates, you will have 1 mole of H+ and 1 mol of A-. concentration of H+ and A- will be the same.

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