Acid-Base Equilibrium

[Charles Darwin]

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Below are some common indicators listed with the pH associated with their color changes. Which of the following would be useful for determining the equivalence point (to within 1%) in a titration of 1 N strong acid with a strong base?

a. thymol blue (pH 8-9)
b. litmus (pH 6-8)
c. methyl red (pH 4-6)
d. all of the above



Answer: D

Can someone explain to me why it's not B?

 

[Raiden2012]

I think it has to do with the 1% +- of pH 7. So the range has to incorporate pH between 5 and 9

 

[shaboobly]

I think it has to do with the 1% +- of pH 7. So the range has to incorporate pH between 5 and 9

yea... the 1% means it can be 1% away from 7 on either side.

 

[Charles Darwin]

So it's 1% of 1.0*10^-7 which is (1*10^-2)(1*10^-7) = 9 or (1*10^-7)/(1*10^-2) = 5? That would make more sense. I was assuming the 1% meant the indicator had to change color between a pH of 6.93 and 7.07.

 

[sciencebooks]

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[sciencebooks]

So it's 1% of 1.0*10^-7 which is (1*10^-2)(1*10^-7) = 9 or (1*10^-7)/(1*10^-2) = 5? That would make more sense. I was assuming the 1% meant the indicator had to change color between a pH of 6.93 and 7.07.

You know, I got D not thinking too deeply of the one percent, but I guess the wording is sort of ambiguous. I went with the rule that pH indicators are usually +/- 1.

 

[Raiden2012]

.

 

[shaboobly]

If it helps, remember that the equivalence point encompasses a pretty broad range of pH. That long vertical line so it had to be +-1% of H+

pretty sure the eq point is at one specific point in the pH when the number of moles of base is = to the number of moles of acid which implies a single point, not a broad range.

 

[V5RED]

So it's 1% of 1.0*10^-7 which is (1*10^-2)(1*10^-7) = 9 or (1*10^-7)/(1*10^-2) = 5? That would make more sense. I was assuming the 1% meant the indicator had to change color between a pH of 6.93 and 7.07.

Those calculations give you the pH for a solution which is either 100 times more acidic or 100 times more basic than neutral. That would be +/-10,000%.(recall that each integer change in pH represents a tenfold change in acidity)

+/- 1% of the acidity gives a range of 6.995-7.004
-log(10^-7 +/- .01*10^-7)

+/- 1% of the actual pH # gives a range of 6.93-7.07

I am not sure what the 1% refers to, but if they meant either acidity or pH it makes their answer seem odd.

The general trend of using indicators that have a pKa within one unit of the pH of the solution would indicate that your original answer is the best one especially since they say that want such a precise measure.

 

[Raiden2012]

pretty sure the eq point is at one specific point in the pH when the number of moles of base is = to the number of moles of acid which implies a single point, not a broad range.

yep you're right. My bad

 

[Charles Darwin]

Those calculations give you the pH for a solution which is either 100 times more acidic or 100 times more basic than neutral. That would be +/-10,000%.(recall that each integer change in pH represents a tenfold change in acidity)

+/- 1% of the acidity gives a range of 6.995-7.004
-log(10^-7 +/- .01*10^-7)

+/- 1% of the actual pH # gives a range of 6.93-7.07

I am not sure what the 1% refers to, but if they meant either acidity or pH it makes their answer seem odd.

The general trend of using indicators that have a pKa within one unit of the pH of the solution would indicate that your original answer is the best one especially since they say that want such a precise measure.

Exactly, that was my assumption as well. Chalk one up to ambiguous question-making...

 

[V5RED]

Was this an AAMC question? I ask because even though I prefer your answer to theirs, if that is how they want to call it, that would be useful info for the MCAT.

 

[Charles Darwin]

Was this an AAMC question? I ask because even though I prefer your answer to theirs, if that is how they want to call it, that would be useful info for the MCAT.

No it was from my school's website on acid-base equilibrium. I highly doubt AAMC would put forth a question that tricky.

 

[Singhp03]

Those calculations give you the pH for a solution which is either 100 times more acidic or 100 times more basic than neutral. That would be +/-10,000%.(recall that each integer change in pH represents a tenfold change in acidity)

+/- 1% of the acidity gives a range of 6.995-7.004
-log(10^-7 +/- .01*10^-7)

+/- 1% of the actual pH # gives a range of 6.93-7.07

I am not sure what the 1% refers to, but if they meant either acidity or pH it makes their answer seem odd.

The general trend of using indicators that have a pKa within one unit of the pH of the solution would indicate that your original answer is the best one especially since they say that want such a precise measure.

their answer is weird, we use indicators in lab all the time, the rational we use in lab suggest the (most practical) answer is B

 

[Temperature101]

their answer is weird, we use indicators in lab all the time, the rational we use in lab suggest the (most practical) answer is B

That is what I thought....I am gonna go with B as my final answer.