i just got burned

[thebillsfan]

on a really easy titration question. this is what i do know:

the equiv point is defined as the pH when the equivalents of acid and base are equal.

the half equiv is defined as the pH when the [acid] = [its conj base]

now, the question i missed was basically asking, what's the pH of the solution when the quantity of hydronium ion equals the quantity of hydroxide ion?

i was deciding btw whether this referred to the half equiv pt or the equivalence pt and decided on half equivalence point. this was obviously wrong. by asking for the pH of equal [H30] and [OH-], they were asking for the equivalence point. my question is, if the equivalence pt doesnt always have to be 7, then how can the equivalence point be defined as "the pH of the solution when the quantity of hydronium ion equals the quantity of hydroxide ion?" wouldnt equal [H30] and [OH-] necessarily have a pH of 7?

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[RogueUnicorn]

if you're titrating strong acid with strong base then the eq. point is always 7.. perhaps that's what was implied by the question.

 

[thebillsfan]

yep, youre right that they were talking about a strong acid/strong base titration. so lets say they were talking about a strong acid/weak base titration in which the equiv point pH is greater than 7. at the equivalence point, would the [OH-] still equal [H30]? or is the only reason they could phrase the question the way they did because it was a strong acid/strong base?

 

[ratatat]

yep, youre right that they were talking about a strong acid/strong base titration. so lets say they were talking about a strong acid/weak base titration in which the equiv point pH is greater than 7. at the equivalence point, would the [OH-] still equal [H30]? or is the only reason they could phrase the question the way they did because it was a strong acid/strong base?

Nah at the equivalence point in a weak acid/strong base or strong acid/weak base titration, there wouldn't be equal [] of H3O+ and OH-.

So in a strong-strong titration, (say HCl + NaOH), the salt formed is NaCl. None of the salts hydrolyze and the pH is 7.

In a weak base-strong acid titration, (say NH3 + HCl), the salt formed is NH4Cl, or NH4+ and Cl-. Since Cl- is from the strong acid, it doesn't hydrolyze. However, NH4+ will hydrolyze with water to reform the weak base.

NH4+ + H2O ---> NH3 + H3O+

The solution is then acidic. That's why the pH of a weak base-strong acid equivalence point is on the acidic side. I believe the reason the NH4+ hydrolyzes is because NH3 is not a very strong base so its conversion to NH4 cannot go to completion, so some of the NH4 is converted back to NH3.

 

[thebillsfan]

im just trying to figure out how they couldve worded that question if they wanted to refer to the HALF equivalence point. basically, what are the relative conc. of [H30] and [OH] when we're at the 1/2 equiv pt? i know it dep on the type of titration, but lets say weak acid strong base. since we're still at the acidic stage at the 1/2 equiv pt, could we say the [H30]=2[OH-]? or is there no definite relationship? i thought i knew titration pretty well but this one really threw me off.

 

[ratatat]

Hmmm...I think at the half equivalence point, the pH is the pKa of the solution that is being titrated right?

So, I'm not exactly sure, but they could have asked when is the pH of the solution at the the pKa value? Then you could have said the half-equivalence point?

But yeah, like you said, I think the half equivalence point would vary for each acid/base. I could be wrong though, I'm not an expert on this.

 

[ratatat]

Also, I'm a Bills fan too...GO BILLS tonight..

 

[RogueUnicorn]

Hmmm...I think at the half equivalence point, the pH is the pKa of the solution that is being titrated right?

So, I'm not exactly sure, but they could have asked when is the pH of the solution at the the pKa value? Then you could have said the half-equivalence point?

But yeah, like you said, I think the half equivalence point would vary for each acid/base. I could be wrong though, I'm not an expert on this.

all true

 

[thebillsfan]

oh my god, ive been so into mcat that i completely forgot about the bills tonight. thanks for reminding me, im gonna have to take a study break and watch (the entire) game.

alright, let me try my hand at one more titratino question...at the half equiv pt, [HA] = [A-]. i guess whats causing my fundamental confusion is that it seems, according to this equation that at the half equiv point the the conc of acid and base should be equal, but i know that the EQUIVALENCE point is when the acid and base conc are equal. why i realize this is extremely elementary, but i missed such an easy titration question for a reason and i'm trying to figure out why.

 

[pnoybballin]

oh my god, ive been so into mcat that i completely forgot about the bills tonight. thanks for reminding me, im gonna have to take a study break and watch (the entire) game.

alright, let me try my hand at one more titratino question...at the half equiv pt, [HA] = [A-]. i guess whats causing my fundamental confusion is that it seems, according to this equation that at the half equiv point the the conc of acid and base should be equal, but i know that the EQUIVALENCE point is when the acid and base conc are equal. why i realize this is extremely elementary, but i missed such an easy titration question for a reason and i'm trying to figure out why.

Just remember that at the half-equivalence point, the conc of the Acid and its CONJUGATE base are equal and thus the pH = pKa. And at the Equivalence point, the Moles Acid = Moles Base.