The student Louise continued the analysis of acetic acid using half - neutralization technique. She titrates this acetic acid solution using 0.984 M NaOH titrant solution. When volume of NaOH reaches 14.34 mL, she stopped titration. The averaged pH value is measured to be 4.75.  Determine the acid dissociation constant Ka.

Chemistry: The Molecular Science
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ISBN:9781285199047
Author:John W. Moore, Conrad L. Stanitski
Publisher:John W. Moore, Conrad L. Stanitski
Chapter15: Additional Aqueous Equilibria
Section: Chapter Questions
Problem 93QRT: When 40.00 mL of a weak monoprotic acid solution is titrated with 0.100-M NaOH, the equivalence...
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The student Louise continued the analysis of acetic acid using half - neutralization technique. She titrates this acetic acid solution using 0.984 M NaOH titrant solution. When volume of NaOH reaches 14.34 mL, she stopped titration. The averaged pH value is measured to be 4.75. 

Determine the acid dissociation constant Ka. 

Expert Solution
Step 1

The reaction between acetic acid and NaOH is

NaOH + CH3COOH --------> CH3COONa + H2O

 

Step 2

Since the titration is done till half neutralisation point.

Hence it means only half of the acid has reacted.

Hence concentration of salt CH3COONa formed = initial concentration of CH3COOH taken / 2                     ( i.e half of acid)

And concentration of CH3COOH remaining = initial concentration of CH3COOH / 2                                         ( i.e half of initial)

Hence at half neutralisation point, also known as half equivalence point,

the concentration of CH3COOH i.e acid remaining = the concentration of CH3COONa i.e conjugate base salt of acid

Since the solution has both acid and its conjugate base. Hence it will act as a buffer solution.

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