NS FL 2 C/P Q10

[Thoth]

Relevant information:

1 L samples from each broth were then inoculated with A. aceti grown in 5%, 10%, or 15% bacterial concentration growth media. The acidity of each broth sample was evaluated by acid–base titrationwith a standardized solution of 0.1 N sodium hydroxide, using phenolphthalein as an indicator. The acetic acid content of the fermentation products were measured 24, 48, and 72 hours after inoculation and reported as titratable acidity (%TA) in Table 2.X

Table 2 Titratable acidity percentage (%TA) in acetic acid fermentation broths

Question:

Which of the following most closely approximates the pKa of phenolphthalein?


A.
2.4

B.
4.8

C.
6

D.
9.3

Explanation:

D is correct. Indicators function in acid-base titrations to identify, via color change or a similar mechanism, that an expected pH, and thus the titration endpoint, has been reached. In order to function in this way, an indicator must undergo a color change near the desired pH. This typically occurs because of a reversible change in the protonation state of the indicator. It is desirable then that the pKa of a chosen indicator be within ± 1 unit of the target pH. For the titrations performed in the study of acetic acid, a weak acid, and sodium hydroxide, a strong base, the end point of the titration will occur at a pH greater than 7. Of the possible pKa values given as answer choices, only D could function well as an indicator for a pH in this range.



A, B, C: These pKa’s would be too low to serve as an indicator of a solution pH greater than 7.

I picked B, because based on the table that is where the ranges fell. I don't understand why the titrant matters. The goal was to quantify the concentration of acetic acid, not sodium hydroxide. Anyone?

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[Mr. otcoD]

The whole point of a titration is to find the equivalence point of the reaction between the acid and the base. A SA and a SB usually has the point around 7. IN this case, you are dealing with a WA and a SB (Acetic Acid and NaOH). So when you react it, you're gonna obtain an equivalence point above 7, around the range of 9-11 in my opinion.
For the Indicator, how can you even tell if a reaction has even happened? In a lot of cases you could be dealing with colorless substances. When the exchange of H+ and OH- are equivalent with each other, it can give off a substance that can react with the indicator to tell you when the reaction has happened. You've seen the photos where theres like a pink substance in an Erlenmeyer flask right? The reaction turned pink because the indicator was right at the equivalence point to tell you when the exchange between H+ and OH- were the same or at least similar.
Overall, its just the fact where you want the indicator to be +- 1 to the pH of the equivalence point. So I'm thinking the answer should be 9.3

 

[Thoth]

The whole point of a titration is to find the equivalence point of the reaction between the acid and the base. A SA and a SB usually has the point around 7. IN this case, you are dealing with a WA and a SB (Acetic Acid and NaOH). So when you react it, you're gonna obtain an equivalence point above 7, around the range of 9-11 in my opinion.
For the Indicator, how can you even tell if a reaction has even happened? In a lot of cases you could be dealing with colorless substances. When the exchange of H+ and OH- are equivalent with each other, it can give off a substance that can react with the indicator to tell you when the reaction has happened. You've seen the photos where theres like a pink substance in an Erlenmeyer flask right? The reaction turned pink because the indicator was right at the equivalence point to tell you when the exchange between H+ and OH- were the same or at least similar.
Overall, its just the fact where you want the indicator to be +- 1 to the pH of the equivalence point. So I'm thinking the answer should be 9.3

So let me make sure I understand this - because it is a weak acid, it takes a lot of OH- to dissociate it enough to reach the equivalence point of OH-. Therefore, because it uses a lot of OH-, that means that it makes the solution super basic. Yes?

 

[Mr. otcoD]

So let me make sure I understand this - because it is a weak acid, it takes a lot of OH- to dissociate it enough to reach the equivalence point of OH-. Therefore, because it uses a lot of OH-, that means that it makes the solution super basic. Yes?

Because it is a weak acid, that means that it won't dissociate completely. The H will come off from the acid, but it will almost immediately go back to the conjugate base.
What the OH- does is that it stabilizes the H+ from going back to the acid by binding do it. You will need the power from OH- (a strong base) to counteract the strong power of the H+ to go back to the weak acid, hence the amount of H+ in the overall solution is lower. pH is a measure of how much H is in the solution. If theres LOW H+, that means theres HIGH OH-, and that means the pH is ABOVE 7

 

[aldol16]

OP, you should notice first that the values given in the table are not pH measures. They are acidity content by percentage. Also, knowing that the end point for titrating a weak acid with a strong base is above 7 is something that's good to know. One of your gen chem labs probably focused on this very concept.

 

[Thoth]

OP, you should notice first that the values given in the table are not pH measures. They are acidity content by percentage. Also, knowing that the end point for titrating a weak acid with a strong base is above 7 is something that's good to know. One of your gen chem labs probably focused on this very concept.

Probably...10 years ago..

#old