diprotic titrations

[nickelbackfan]

Question out of TBR

For which of these titrations is the pH at equivalence the same, regardless of concentration?

A- the titration of a weak acid by a weak base
B- The titration of a strong acid by a weak base
C-The titration of the first proton of a weak diprotic acid by a strong base
D-The titration of the second proton of a weak diprotic acid by a strong base



Answer is C, I dont really understand why it would affect the second proton but not the first?

Thanks

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[nickelbackfan]

bump

 

[Sandstorm]

bump

First proton has a pretty large Ka value so it is assumed it will pretty much completely dissociate regardless. Second proton has a much smaller Ka so it is concentration dependent

 

[Singhp03]

in strong dissociation, equi not dependent on concentration.....for example (if they mentioned a strong diprotic acid) H2SO4, the first H is strongly acidic and thus dissociates completely...the 2nd H and its resulting equi point, would depend upon concentration. furthermore, I understand that titrants need to be strong acids or bases, which cancels ans A and B, leaving choice C or D, of which C is the proper ans.

 

[allatrope]

Although I understand what you are saying here about the first proton having a very large Ka and therefore deprotonating first, the TBR answer is slightly different and I honestly don't quite understand it.

"The pH at equivalence when the first proton of a weak diprotic acid has been removed is the average of pKa1 and pKa2. Because the pKa values do not change, the pH at equivalence does not change, so the same pH at equivalence is observed. The best answer is C. The second proton of a diprotic acid is affected by the concentration, so choice D is eliminated.

Of course the pKa values do not change, they don't change for the second proton either. How does this answer the question? Anyone?

 

[dudewheresmymd]

A- the titration of a weak acid by a weak base (never occurs in real life, cross out)
B- The titration of a strong acid by a weak base (at equivalence you have strong acid's conjugate (a weak base) and the weak base that's titrating so ph at equivalence would depend on concentration of both
C-The titration of the first proton of a weak diprotic acid by a strong base (TBR is referring to graph on page 3.16 figure 5-14, at first equivalence point ph= pka1+pka2/2, pkas do NOT depend on concentration, so this is the correct answer)
D-The titration of the second proton of a weak diprotic acid by a strong base (this is the second equivalence point in figure 5-14), notice how now the ph is not given by the average of the pkas, rather ph titrant +pka / 2 as an approximation similar to a weak acid titrated by a strong base (look at table 5.2 p. 306 at equivalence for the formula) B/c ph of titrant would determine the ph of the SECOND proton, this is not the best answer.

Don't worry this question is just testing the application of the formula (pka1+pka2)/2 =ph at the equivalence point in the titration of a diprotic acid by a strong base, similarly in a triportic acid the ph at the second equivalence point = (pka2+pka3)/2

 

[KoalaT]

I'm very confused on this as well and none of these answers seem to be explaining it appropriately. As well, EK Gchem #860 seems to complete contradict this answer. Which is right?

The Ka1 for H2C03 is 4.3x10^-7. The Ka2 for the H2C03 is 5.6x10^-11. Which of the following depends upon the concentration of acid and conjugate base?
A. The pH of the first equivalence point
B. The pH of the first half equivalence point
C. The pH of the second half equivalence point
D. The pKa1

It simply states that the answer is A because "The equivalence points depend on concentration".

 

[JoeCaputo]

I'm very confused on this as well and none of these answers seem to be explaining it appropriately. As well, EK Gchem #860 seems to complete contradict this answer. Which is right?

The Ka1 for H2C03 is 4.3x10^-7. The Ka2 for the H2C03 is 5.6x10^-11. Which of the following depends upon the concentration of acid and conjugate base?
A. The pH of the first equivalence point
B. The pH of the first half equivalence point
C. The pH of the second half equivalence point
D. The pKa1

It simply states that the answer is A because "The equivalence points depend on concentration".

At the half equivalency points, the concentrations of acid and conj. base will be in a 1:1 ratio, so the concentration itself does not matter. The conjugate base of H2CO3 is HCO3-. The more H2CO3 you start with, the more it would effect pH at the equivalence point because the concentration of NaHCO3 will be higher in the end. Unless you are forming a neutral salt and water, the pH at equivalency will depend on concentration. I am not sure about the first question from the OP though.