Understanding dissociation constants

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Discussion Overview

The discussion revolves around the concept of dissociation constants (Ka) in the context of acid-base chemistry, specifically addressing the definitions and implications of the terms used in the expression for Ka. It includes theoretical considerations and clarifications regarding equilibrium concentrations.

Discussion Character

  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the term "[HA]" in the Ka expression refers to its initial or equilibrium concentration.
  • Another participant clarifies that all concentrations in the expression are equilibrium concentrations, noting that the initial concentration of acid is represented as the sum of [HA] and [A-].
  • A further inquiry is made regarding the necessity of including both [H+] and [A-] in the numerator of the Ka expression, suggesting that it might be sufficient to use either [A-] or [H+] alone.
  • A response emphasizes that both [H+] and [A-] are products of the dissociation reaction, and that changing the concentration of either will shift the equilibrium, thus necessitating their inclusion in the Ka expression.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of including both [H+] and [A-] in the Ka expression, indicating a lack of consensus on this aspect of the discussion.

Contextual Notes

The discussion does not resolve the implications of using initial versus equilibrium concentrations, nor does it clarify the potential consequences of omitting one of the terms in the Ka expression.

sodium.dioxid
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Ka = [H+][A-]/[HA]

Is the term "[HA]" referring to its initial concentration or its equilibrium concentration?
 
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All concentrations are equilibrium concentrations. Initial concentration of acid (sometimes called formal or analytical) is [HA]+[A-].
 
OK, another question: why is it necessary to have both [H+] and [A-] on the numerator? It seems like we could have done just as fine using the definition "Ka=[A-]/[HA]" or "Ka=[H+]/[HA]" instead of "Ka=[H+][A-]/[HA]". Is this conventional or am I missing something?
 
Last edited:
As both H+ and A- are products of the reaction changing concentration of either one shifts the equilibrium, so you need both.
 

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