How do you determine charge from pH and pKa?

[the_fella]

I'll copy/paste the question:

Given the following information, what is the charge of isoleucine at a pH of 9.8?

pKa of COOH = 2.4
pKa of NH3 = 9.8

A) -1
B) -1/2
C) 0
D) +1/2

It says the answer is B. But Idk how they got that. When the pH is greater than pKa, it's deprotonated, so the COO- group should be deprotonated and thus have a -1 charge. But what about the NH3 group? Idk what happens when pH=pKa.

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[Cawolf]

When the pH = pKa the solution is a buffer, and therefore half of the amino groups are protonated and half are deprotonated.

The acid group will be deprotonated - not protonated, making it's charge -1. The pH is much greater than the pKa.

The amino group will be half protonated (+1) and half deprotonated (0) for a net of "+0.5".

This works out to a total of -0.5.

 

[the_fella]

Fantastic! Thank you so much.

 

[Labrat07]

The amino group will be half protonated (+1) and half deprotonated (0) for a net of "+0.5".

This works out to a total of -0.5.

I understand what you're saying here but there's something that bother me. What you're saying the amino protonate form (NH3) = +1 and NH2 = 0. What's about the protonate form of Carboxylic COOH = +1 but COO = -1??? What am I missing here?

Is it because of diprotic/monoprotic principles?? So NH3 has 2 pka??

 

[EParker37]

I understand what you're saying here but there's something that bother me. What you're saying the amino protonate form (NH3) = +1 and NH2 = 0. What's about the protonate form of Carboxylic COOH = +1 but COO = -1??? What am I missing here?

Is it because of diprotic/monoprotic principles?? So NH3 has 2 pka??

I was fairly certain that the protonated form R-COOH has an overall neutral charge, while R-NH3 is indeed a +1 charge. So we end up with COOH = 0 and COO = -1, NH3 = +1, NH2 = 0

 

[Labrat07]

Ah, that make a lot of sense now. Now I understand. Thanks

 

[EParker37]

Ah, that make a lot of sense now. Now I understand. Thanks

Happy to help, and yeah, I forget about the R group on amino acids sometimes too haha.

 

[Cawolf]

Happy to help, and yeah, I forget about the R group on amino acids sometimes too haha.

The R group is neutral here.

I understand what you're saying here but there's something that bother me. What you're saying the amino protonate form (NH3) = +1 and NH2 = 0. What's about the protonate form of Carboxylic COOH = +1 but COO = -1??? What am I missing here?

Is it because of diprotic/monoprotic principles?? So NH3 has 2 pka??

@EParker37 answered your question, but I suggest reviewing your functional groups to make it more clear. To be successful on the MCAT, these are concepts you should be very comfortable with - not memorizing.

 

[the_fella]

I understand what you're saying here but there's something that bother me. What you're saying the amino protonate form (NH3) = +1 and NH2 = 0. What's about the protonate form of Carboxylic COOH = +1 but COO = -1??? What am I missing here?

Is it because of diprotic/monoprotic principles?? So NH3 has 2 pka??

When the pH is lower than the pKa, the species is protonated. Conversely, when the pH is higher than pKa, it's deprotonated. I think you're confused by the way this question is worded.

pH is 9.8

pKa of COOH = 2.4 (this means the COOH group is deprotonated and is actually COO-) (they write COOH group b/c that's the convention, I believe). This gives us a charge of -1.

pKa of NH3 = 9.8 (this is equal to the pH, so it's half protonated, half deprotonated). So we have -1 + 1/2. That leaves us with -1/2. Does this make sense, or have I muddled things more? Haha.

 

[EParker37]

The R group is neutral here.

Right, I was just mentioning them so that the calculation of formal charge on the functional groups would make more sense.

 

[Labrat07]

The R group is neutral here.



@EParker37 answered your question, but I suggest reviewing your functional groups to make it more clear. To be successful on the MCAT, these are concepts you should be very comfortable with - not memorizing.

You're absolutely right now that I pay attention to it. It seem that I have gained a bad habit of memorizing over the last 2 years. Thank you for noticing that.

Does anyone know some effective ways to fix this?? or just be mindful, slow it down and connect subjects?

When the pH is lower than the pKa, the species is protonated. Conversely, when the pH is higher than pKa, it's deprotonated. I think you're confused by the way this question is worded.

pH is 9.8

pKa of COOH = 2.4 (this means the COOH group is deprotonated and is actually COO-) (they write COOH group b/c that's the convention, I believe). This gives us a charge of -1.

pKa of NH3 = 9.8 (this is equal to the pH, so it's half protonated, half deprotonated). So we have -1 + 1/2. That leaves us with -1/2. Does this make sense, or have I muddled things more? Haha.

I understood this part. Thanks though

 

[Cawolf]

Does anyone know some effective ways to fix this?? or just be mindful, slow it down and connect subjects?

Everyone studies differently, but I personally take the time to slow down and think about why things are happening. This is helpful when you are trying to apply your knowledge to novel problems that are not just memory exercises. This is the type of problem I believe the MCAT authors tend to like the best.

 

[Labrat07]

@Cawolf
Gold advice. Thank you very much. Really appreciate it.

 

[the_fella]

It's been said before, but the best advice I can give you is to do as many official practice problems as possible.

 

[shefv]

I am confused

I was trying to visualize this with the plot

I calculated pI = 6.1, I know that if pH is below this, then it is positively charged and if pH is above this, then it is negatively charged but I am confused between -1/2 and +1 thing

Is it always +1/2 and -1/2 at the two half equivalence points (meaning at the point of pKa1 and pKa2)?

 

[EParker37]

I am confused

I was trying to visualize this with the plot

I calculated pI = 6.1, I know that if pH is below this, then it is positively charged and if pH is above this, then it is negatively charged but I am confused between -1/2 and +1 thing

Is it always +1/2 and -1/2 at the two half equivalence points (meaning at the point of pKa1 and pKa2)?

At the half equivalence you end up with a buffer. So both the amine and carboxyl group are half protonated and half deprotonated at their respective points. Since the pH = the pKa of the amine, it is going to have a half overall charge, given that it is +1 and 0 for the respective species, it will be +0.5. Since the pH is significantly higher than the pKa of the carboxyl, it will be fully de-protonated, giving the overall -1 charge. Combine the two to find the charge for the entire AA at a given pH, in this case -1/2.

For the other part of your question, unless a group has a different charge, it will most likely end up being either +1/2 or -1/2 for a given half equivalence point. But you don't need a titration plot to figure out this problem.