equivalence point TBR

[Meredith92]

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A few questions on equivalence:

- TBR says we can estimate the equivalence point by averaging the pKa of the acid and the pH of the titrant. But it also says that the equivalence point changes based on concentration of the acid. Is this just not taken into account in this short cut??

- I'm not very clear in general how to estimate the pH at equivalence with a weak acid titrated with a strong base. Should we use the pKa +pH average? Should we use pOH=1/2pKb-1/2log[A-]? How do you guys normally do it?

- Can someone explain table 5.3 on page 306 in the chem 1 book? (it does pH calculations) Why are they using mL instead of moles in the henderson-hasselbach equation?

THANK YOU!! I just wanna throw it how thankful I am for sdn.. "mcat study question Q&A" keeps me excited about studying for the mcat.. its helpful (and fun!) to discuss questions!! 😍

 

[Meredith92]

one more quick question!
when a pH is within +/- 1 unit from the pKa of an indicator is it always a mix of color? for instance, if methyl red has a pKa of 5.21, and its protonated form is red, while its deprotonated form is yellow, does that mean between 4.21 and 6.21 it is a mix of yellow and red?

 

[BryanNextStep]

(1) estimate the equivalence point by averaging the pKa of the acid and the pH of the titrant. But it also says that the equivalence point changes based on concentration of the acid.

(2) how to estimate the pH at equivalence with a weak acid titrated with a strong base. Should we use the pKa +pH average? Should we use pOH=1/2pKb-1/2log[A-]? How do you guys normally do it?

(1) It sounds like you think those two sentences are contradictory. But as the concentration of the acid goes up, the pH of the acid goes down. So they both could be true.

(2) When you hit the equivalence point, the weak acid has entirely been converted to its conjugate base. The strong base would be entirely converted to its salt, which has negligible impact on the pH. So I'd focus any calculations on the conjugate base of the weak acid.

For example, titrating CH3COOH with NaOH. At the equivalence point, all of the OH's and H's have neutralized into water. So all you're left with is the spectator ion Na+ and the acetate base: CH3COO-

To calculate the pH at that point, you'd need to the concentration of CH3COO- and its Kb. That's how the problems are usually written (sometimes they give you the Ka instead and you have to divide it into Kw to get Kb)

Hope this helps 🙂

 

[BryanNextStep]

for instance, if methyl red has a pKa of 5.21, and its protonated form is red, while its deprotonated form is yellow, does that mean between 4.21 and 6.21 it is a mix of yellow and red?

Remember that even one point on the pH scale means a tenfold increase (or decrease) in the amount of [H+] present.

So if you're exactly at pH 5.21, then the thing is gonna look orange, since it's equal amounts of red and yellow.

But by the time you get to 4.21, you've increased the [H+] by 10 times. That means you're going to have a solution that is like 90% red and 9% yellow - which is gonna look basically red.

Going the other way, at pH 6.21, you'll be at 9% red and 90% yellow, so it'll mostly just look yellow.

 

[Meredith92]

Thank you! I realize that first question was pretty stupid now lol

if anyone can answer this that would be great:
- Can someone explain table 5.3 on page 306 in the chem 1 book? (it does pH calculations) Why are they using mL instead of moles in the henderson-hasselbach equation?

 

[BerkReviewTeach]

- TBR says we can estimate the equivalence point by averaging the pKa of the acid and the pH of the titrant. But it also says that the equivalence point changes based on concentration of the acid. Is this just not taken into account in this short cut?

It is taken into account with the shortcut method. In one case it's stated in the way you learned it in general chemistry and the other it's stated a way that's better for a multiple-choice exam like the MCAT.

- I'm not very clear in general how to estimate the pH at equivalence with a weak acid titrated with a strong base. Should we use the pKa +pH average? Should we use pOH=1/2pKb-1/2log[A-]? How do you guys normally do it?

The shortcut method is SO much faster, but it's off by about 0.15. On a test like the MCAT, that's not a big deal, so I'd use the shortcut method and save time.

- Can someone explain table 5.3 on page 306 in the chem 1 book? (it does pH calculations) Why are they using mL instead of moles in the henderson-hasselbach equation?

I think you must have an older book, because Tables 5.1 and 5.2 are on page 306. If you are talking about the pH calculation table, then it lists mL because that's what the x-axis in a titration lists. So if it takes 50mL of base to reach equivalence for instance, then when you've added 20 mL of base, you are 40% titrated and have 60% to go. This means that the ratio of conjugate based formed to conjugate acid remaining is 40%-60%. In terms of the HH equation (applicable during the middle part of the titration when pH is relatively constant), you could put in (moles CB/moles CA), (equivalents CB/equivalents CA), or in this case (mL OH- added)/(mL left to go to reach equivalence). All of them will give you the same ratio.

(2) When you hit the equivalence point, the weak acid has entirely been converted to its conjugate base. The strong base would be entirely converted to its salt, which has negligible impact on the pH. So I'd focus any calculations on the conjugate base of the weak acid.

For example, titrating CH3COOH with NaOH. At the equivalence point, all of the OH's and H's have neutralized into water. So all you're left with is the spectator ion Na+ and the acetate base: CH3COO-

To calculate the pH at that point, you'd need to the concentration of CH3COO- and its Kb. That's how the problems are usually written (sometimes they give you the Ka instead and you have to divide it into Kw to get Kb)

That method (the traditional school method) definitely works, but the problem is that it takes way too much time for a test like the MCAT. The BR shortcut equation is significantly faster and gives you an answer that is very close in about three seconds. Saving time is a godsend on the MCAT and should always be a priority on math questions.