What Went Wrong in My Ethanoic Acid and NaOH Titration Calculation?

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Discussion Overview

The discussion revolves around a titration calculation involving ethanoic acid and sodium hydroxide, specifically addressing the determination of pH after neutralization. Participants explore the implications of the equilibrium constant and the presence of acetate ions during the titration process.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant calculates the pH assuming 0.05 M of acetic acid remains after 50% neutralization, using the equilibrium constant Ka but arrives at a pH that does not match provided answers.
  • Another participant points out that the presence of acetate ions cannot be ignored, suggesting that at half titration, the concentrations of acetic acid and acetate are equal.
  • A different participant proposes a revised equilibrium expression that incorporates acetate, leading to a calculated pH of 4.74.
  • One participant suggests using the Henderson-Hasselbalch equation for a simpler solution, noting that at half titration, pH equals pKa.
  • A later reply emphasizes the importance of explicitly writing the equilibrium constant in terms of chemical species to avoid errors in calculations.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to the titration calculation, with some advocating for the inclusion of acetate and others focusing on the initial assumptions made in the calculations. No consensus is reached on the correct method or final pH value.

Contextual Notes

Limitations include potential misunderstandings of the equilibrium expressions and the role of acetate ions in the titration process. Some participants may have overlooked the implications of the half-titration point on pH calculations.

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Homework Statement


Screen Shot 2016-07-05 at 5.12.39 PM.png


Homework Equations

The Attempt at a Solution


If 50% of the acid has been neutralised, then there is 0.05 M of acetic acid left.Using Ka
$$ 1.8*10^-5 = \frac {x^2}{0.05-x} $$
I get x = 0.00094
-log(0.00094) =3.02687 =pH
This is not one of the answers provided. I think I interpreted the question wrong. Can someone explain to me what I did wrong?

Cheers!
 
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You can't ignore acetate presence.

Hint: at half titration [CH3COOH] = [CH3COO-]
 
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I see, then the equilibrium is:

$$ 1.8*10^{-5} = \frac {(0.05+x)*x} {0.05-x}$$

Therefore, pH = 4.74

Answer is B.

Thanks again!
 
Much easier to solve using Henderson-Hasselbalch equation. Not to mention the fact you should remember at half titration pH=pK.
 
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I see, thanks a lot!
 
TT0 said:
If 50% of the acid has been neutralised, then there is 0.05 M of acetic acid left.Using Ka
$$ 1.8*10^-5 = \frac {x^2}{0.05-x} $$
I get x = 0.00094
-log(0.00094) =3.02687 =pH
This is not one of the answers provided. I think I interpreted the question wrong. Can someone explain to me what I did wrong?

Cheers!

If you had written the equation for the equilibrium constant out explicitly in terms of chemical species you would probably not gone wrong in the way you did.
 
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