TBR Titration Table Passage

[greenseeking]

Hi I was wondering if anybody could please help me with this question.

The Berkeley Review Titration Table Passage Question 11:

Given: 25 ml of a 0.1 M sample of the acid is used. Titrant is a strong base 0.05M and it takes 50 ml to reach the equivalence point.
It takes 14 ml of base (titrant) to make the PH of the solution 7.

Question: when the PH is 7.00, what is true about the ratio of weak acid to conjugate base in solution?

Anwer: The weak acid exceeds the conjugate base in the ratio of 2.53:1.

TBR's answer explanation says that the ratio of acid to base is 36:14 or about 2.53:1... Where did the 36 come from? I get that it's from 50-14 but why isn't it 25ml? There's only 25 ml of acid in the solution.... And why are we supposed to use the ratios of the volumes?

Also, another way to finding the answer is to use the henderson-hasselbach equation but I don't see how they did this.

Any help would be appreciated. Thank you!

---

Members don't see this ad.

 

[chiddler]

Need the pKa of the compound being titrated to answer properly.

As you wrote, can use H.H equation but it is

pH = pKa + log(A-/HA)

need more info.

 

[greenseeking]

oh I see- Yeah, they give you the pka of the compound which is 7.44. so it would be -.44=logB/A which would give you the ratio.

Thanks chiddler!!

Do you have any idea what they were talking about when they did the volumes ratio of 14:36?

 

[chiddler]

sorry i'm not sure what is meant by that.

 

[rjosh33]

oh I see- Yeah, they give you the pka of the compound which is 7.44. so it would be -.44=logB/A which would give you the ratio.

Thanks chiddler!!

Do you have any idea what they were talking about when they did the volumes ratio of 14:36?

You can also set up the log( [HA]/[B-]) portion of the HH equation as a set of ratios once you know the equivalence volume (which is the first thing you should usually figure out anyway in titration calculations). You take the amount added and divide by the equivalence volume.

So, using TBR's example, the ratio of [HA] to [B-] is (36/50) / (14/50) = 2.57 (not sure where they got 2.53). You use 36/50 for the acid instead of 18/25 merely for convenience. What's important is the fractional portion of acid that is left in solution after the addition of titrant. 36/50ths (or 18/25ths) are left, and so this is the number representing the [HA] part of the HH equation.

 

[greenseeking]

thanks rjosh33!