Henderson Hasselbalch equation Where did I go wrong?

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Discussion Overview

The discussion revolves around the application of the Henderson-Hasselbalch equation to a homework problem involving the preparation of a buffer solution. Participants explore the calculations needed to determine the volume of hydrochloric acid required to achieve a specific pH in a sodium acetate solution.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • The original poster presents a calculation to determine the volume of 0.1M HCl needed, using the Henderson-Hasselbalch equation and providing their steps.
  • Some participants suggest that the moles of salt and acid should remain constant throughout the process.
  • One participant points out the importance of considering the protonation reaction, indicating that adding strong acid increases the amount of acetic acid while decreasing the amount of acetate.
  • There is a request for further clarification on the calculations and concepts involved, indicating a lack of understanding of the initial approach.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the calculations and the underlying chemistry. There is no consensus on the correct approach to the problem, and the discussion remains unresolved.

Contextual Notes

Participants have not fully resolved the assumptions related to the conservation of moles in the buffer system, nor have they clarified the mathematical steps leading to the original poster's conclusion.

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Homework Statement



Q) Calculate the volume of 0.1M HCl you would need to add to 200ml of 0.15M sodium acetate solution to obtain a buffer of pH 4.2.
Ka = 1.74 * 10^-5 moldm-3


Homework Equations



pH = pKa + log [salt]/[acid]


The Attempt at a Solution



Log[salt]/[acid] = pH - pKa

[salt]/[acid] = 0.275

Equation: [salt] = 0.275 [acid]

[salt] = 200*0.15/200+v

[acid] = (0.1*V / 200+v) - (200*0.15/200+v)

Substituting into above equation:

200*0.15/200+v = 0.275(0.1*V/200+v) - 0.275(200*0.15/200+v)

30 = 0.0275v - 8.25

38.25= 0.0275v

V = 38.25/0.0275v

V = 1390 cm^3

This is obviously wrong! Where did I go wrong?
 
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Moles of salt + moles of acid should be constant.
 
Borek said:
Moles of salt + moles of acid should be constant.

I still don't understand... could you explain a bit more please - sorry to bother you, I am just really desperate!
 
Take a look at the protonation reaction. When you add strong acid to acetate amount of acetic acid grows, but amount of acetate goes down.

Could be you tried to account for that, but you did it wrong way.
 

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