TBR acetylsalicylate + HCL question

[allatrope]

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The question is "Which of the following mixtures results in a buffer solution?

A. 1 equiv acetylsalicylic acid + 1 equiv HCl
B. 2 equiv acetylsalicylic acid + 2 equiv HCl
C. 1 equiv acetylsalicylate + 1 equiv HCl
D. 2 equiv acetylsalicylate + 1 equiv HCl

I don't understand why the answer is D, not C. As far as I understand there is 1 molar equivalent of acetylsalicylate for every 1 molar HCl in the balanced equation.

Many Thanks for any help with this

 

[EmergentCough]

If you are making the system, then you are producing a salt (NaCl). Therefore, you have to have more of the acetylsalicylate (weak base) then the HCl so there is still left over weak base to perform the buffering of the system. Therefore the ratio have to be greater that 1:1 between a weak base and strong acid. (Or weak acid and strong base if it were a different question.)

 

[Postictal Raiden]

If you are making the system, then you are producing a salt (NaCl). Therefore, you have to have more of the acetylsalicylate (weak base) then the HCl so there is still left over weak base to perform the buffering of the system. Therefore the ratio have to be greater that 1:1 between a weak base and strong acid. (Or weak acid and strong base if it were a different question.)

Are you saying it is because the Ka of HCl is greater than the Kb of acetylsalicylate there should be more acetylsalicylate to consume the dissociated protons of HCl? If so, then how can we determine that the buffer would require exactly twice as much as HCl?

 

[allatrope]

I just got this. Here we go.

The key term to keep in mind here is buffer solution. A buffer solution is a roughly equal mole mixture of a weak acid and its conjugate base. This ensures that the pH doesn't drastically change even if a strong acid or a strong base are added to the solution. So the goal is to make a buffer solution, for which we need a conjugate pair of weak acid and base.

In this problem we are given HCl, a strong acid and a variety of acetylsalicylic acid/ acetylsalicylate (aspirin variants). After adding 1 equivalent of HCl (will protonate something) with 1 equivalent of acetylsalicylate we have ultimately 1 equivalent of the weak acid with NaCl. To have a buffer solution we need 1 more equivalent of the base in order to have 1 equivalent of the conjugate acid and 1 equivalent of the conjugate base.

That's why by adding 2 equivalents of the base form, the HCl protonates 1 of the equivalents leaving us with equal mole mix of a conjugate base pair, our buffer solution.

 

[gwjib04]

That's why by adding 2 equivalents of the base form, the HCl protonates 1 of the equivalents leaving us with equal mole mix of a conjugate base pair, our buffer solution.

Your explanation is very nice.

TBR Buffers and Titrations Intro says:

2 ways to make a buffer
1: combine a conjugate pair in roughly equal mole portions
2: partially titrate a weak acid with roughly half of an equivalent of strong base, or vice versa

Thanks

 

[allatrope]

Thanks! This was both my first post and my first response to a question.

This question comes up in TBR before the actual chapter of titrations and buffers. I killed so much time trying to figure it out!

Thanks for adding those rules!

 

[Postictal Raiden]

I just got this. Here we go.

The key term to keep in mind here is buffer solution. A buffer solution is a roughly equal mole mixture of a weak acid and its conjugate base. This ensures that the pH doesn't drastically change even if a strong acid or a strong base are added to the solution. So the goal is to make a buffer solution, for which we need a conjugate pair of weak acid and base.

In this problem we are given HCl, a strong acid and a variety of acetylsalicylic acid/ acetylsalicylate (aspirin variants). After adding 1 equivalent of HCl (will protonate something) with 1 equivalent of acetylsalicylate we have ultimately 1 equivalent of the weak acid with NaCl. To have a buffer solution we need 1 more equivalent of the base in order to have 1 equivalent of the conjugate acid and 1 equivalent of the conjugate base.

That's why by adding 2 equivalents of the base form, the HCl protonates 1 of the equivalents leaving us with equal mole mix of a conjugate base pair, our buffer solution.

Hmmm. That makes sense.

So just I know I understand the concept, you are saying that the job of HCl is merely to turn half of the acetylsalicylate (a base) into and acid, so we could have 50/50 mixture of acid/conjugate base. Right?

I was thinking that a 50/50 mixture of HCl/acetylsalicylate would constitute a buffer. I didn't realize that the acid and the base are not conjugate of one another. Good to know, thanks.

 

[allatrope]

That's right, you got it. A buffer solution is a weak acid/base conjugate pair.