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Topic: buffer solution  (Read 772 times)

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Offline Helly

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buffer solution
« on: January 27, 2020, 10:35:48 AM »
Problem 1.
A physician wishes to prepare a buffer solution at pH = 3.82 that if ciently
resists changes in pH yet contains only small concentrations of the buffering
agents. Determine which one of the following weak acids, together with its
sodium salt, would probably be best to use: m-chlorobenzoic acid, Ka = 1.04e-4
; p-chlorocinnamic acid Ka = 3.89e-5 ; 2,5-dihydroxybenzoic acid, Ka = 1.08e-3
; or acetoacetic acid, Ka = 2.62e-4. Explain.

do we use Henderson-Hasselbalch?

Offline AWK

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Re: buffer solution
« Reply #1 on: January 27, 2020, 11:20:17 AM »
HH equation is enough
AWK

Offline Helly

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Re: buffer solution
« Reply #2 on: January 27, 2020, 02:39:09 PM »
My theory is the concentration of acid and its conjugate in their sodium salts is the same. Therefore, protons concentration depends solely on Ka. And find pH of the acids which close to pH 3.82. Is it true?

Offline Babcock_Hall

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Re: buffer solution
« Reply #3 on: January 27, 2020, 02:41:21 PM »
Why do you do that the concentration of acid and its conjugate base are the same?

Offline Helly

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Re: buffer solution
« Reply #4 on: January 27, 2020, 03:08:24 PM »
If not the same then how to count the protons concentration?

The concentration of its conjugate base is depend on the concentration of the sodium

Offline Babcock_Hall

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Re: buffer solution
« Reply #5 on: January 27, 2020, 03:54:51 PM »
Use the H-H equation.  pH is given to you, which is the same as giving you the concentration of H+.

Offline Borek

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Re: buffer solution
« Reply #6 on: January 27, 2020, 04:52:29 PM »
No need to use HH equation, you are not asked to prepare the buffer, just to select which acid will do the best job.

What do you know about buffers? Will a buffer based on pKa 5 acid work OK buffering a pH 3 solution?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline Helly

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Re: buffer solution
« Reply #7 on: March 06, 2020, 03:43:54 AM »
Is the answer 2,5 dyhydroxy benzoic acid?

Offline Babcock_Hall

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Re: buffer solution
« Reply #8 on: March 06, 2020, 07:28:08 PM »
I would first convert the Ka values into pKa values.  What is a critical property of any buffer?

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