Understanding Acids What is the relationship between Ka and Kw?

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Discussion Overview

The discussion revolves around the relationship between the auto-ionization constant of water (Kw) and the acid dissociation constant (Ka), particularly in the context of calculating pH and pOH for strong and weak acids and bases. Participants explore how these constants apply in different scenarios, including the addition of acids to basic solutions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the relationship between Kw and Ka, noting that the equation pOH + pH = 14 is derived from Kw.
  • Another participant clarifies that Kw is equal to 10^-14, which supports the equation pOH + pH = 14, and states that pH = pOH only at neutral conditions.
  • A participant questions whether Kw can be used with weak acids and bases and seeks clarification on this point.
  • Discussion on strong acids indicates that for monoprotic acids, the concentration of protons equals the concentration of the acid, while for weak acids, the concentration of protons is determined using Ka.
  • One participant provides an example of a weak acid and its dissociation, illustrating how to calculate [H+] and subsequently [OH-] using Kw.
  • A question is raised about the implications of adding an acid to a basic solution and how Kw factors into that scenario.
  • A participant mentions that Kw can be used to calculate [H+] from [OH-] and discusses the approach for mixing strong acids and bases, noting limitations near pH 7.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the application of Kw and Ka, with some agreeing on the basic principles while others seek clarification on specific scenarios. The discussion remains unresolved regarding the broader implications of using these constants in different contexts.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the behavior of weak acids and bases, as well as the conditions under which the equations apply. The participants do not fully resolve these nuances.

yougene
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I'm not sure if this belongs in the HW section but I'm not doing HW so I'll post it here.

My book mentions the auto-ionization constant of water( Kw = [H+][OH-] = 1.0 E-7 ) and shows us how we can use it to find the pH if a strong base is added by solving for [H+].

Later on in the book it introduces you to the acid-disassociation constant( Ka = ([H+][A-])/[HA] ). This makes some of the other information seem inconsistent to me.

For example, pOH + pH = 14 is one equation that is given. But this equation was derived using the auto-ionization constant of water. Does this mean pOH = pH = 14 will only work with strong basic and acidic solutions then?
 
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No wonder you're confused - the auto-ionization constant is Kw = [H+][OH-] = 10^-14 which is the same thing as pOH + pH = 14.

Which also means that pOH = 14 - pH and pH = 14 - pOH. So pH=pOH only for pH=pOH=7, neutral.
 
Thanks for the reply
The 7 was a typo.

That doesn't address the fragmentation in my understanding though.

Can I use Kw equilibrium with weak acids/bases? Why?
 
For strong acids, calculating [H+] is easy because the acid fully dissociates. So, for a monoprotic acid like HCl, the concentration of protons is equal to the concentration for acid. For weak acids like HF, the acid does not fully dissociate so concentration of protons is not equal to the concentration of acid. Here, one uses the expression for Ka to solve for the concentration of protons when the weak acid is added to solution. One can then use the [H+] obtained from the equation for Ka to solve for the concentration of hydroxide ions in solution using the expression for Kw.

For example, let's say we have weak acid HA with Ka = 0.01 at a concentration of 1M. When the acid is added to solution, approximately 1% of the acid will dissociate, giving a [H+] of approx. 0.01M. Using this value we can calculate [OH-] ~ 1.0x10-12M.
 
Thanks, that sums it up nicely.

What if an acid is being added to a basic solution? How does Kw play into that type of scenario?
 
Thanks, that looks comprehensive.
 

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