A Point Of Clarification With Acid/Base Ks

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Discussion Overview

The discussion revolves around the relationships and definitions of acid/base equilibrium constants, specifically focusing on pKa, pKb, pH, and pOH. Participants explore the mathematical connections between these concepts and their implications in acid-base chemistry.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about the meanings of pKa and pKb, seeking clarification on how they relate to other concepts like pH and pOH.
  • Another participant provides a basic definition of pX values, stating that they are the negative logarithm of the concentration of the respective species.
  • There is a suggestion that pKa and pKb can be used to compare acid/base strength, though the mathematical relationships remain unclear to some participants.
  • A participant mentions the equation pA + pB = 14, indicating a relationship between pKa and pKb, but this is met with some uncertainty regarding its application.
  • A later reply challenges the notion that pKw is always equal to 14, referencing the water ion product and its dependence on temperature.
  • Some participants discuss the practical use of pKb in calculations, particularly in determining the pH of salts derived from weak acids.
  • There is a question about whether pKb equals pOH at equivalence, indicating a need for further clarification on this point.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement on the definitions and relationships of pKa, pKb, pH, and pOH. Some concepts are accepted as definitions, while others remain contested or unclear, particularly regarding the implications of temperature on pKw.

Contextual Notes

There are unresolved questions about the mathematical relationships between pKa, pKb, pH, and pOH, as well as the impact of temperature on pKw. Participants also exhibit uncertainty in terminology and its correct application in context.

Who May Find This Useful

This discussion may be useful for students and individuals seeking to deepen their understanding of acid-base chemistry, particularly those grappling with the mathematical relationships and definitions of related concepts.

Lancelot59
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Ok, so I understand the relationships between the basic forms of acid/base equilbriums:

KW=(Ka)(Kb)

Kw being the ionization constant of water, with Ka,and Kb being the ionization constants for acids and bases respectively.

Then there's pKW = pH + pOH

However I get confused when I get to pKa and pKb. I can't really wrap my head around what exactly these mean. It's a similar story with pH and pOH. All I know is that they're a numeric measure of acidity or basic properties, and their mathematic definition as the negative log of the concentration of hydronium/hydroxide respectively.

How do pKa and pKb relate back to everything else? Also could I get a better definition of pH/pOH, or is my definition good enough?
 
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Those pX numbers are just what you have read that they are. pX is "negative of the logarithm of the X..." That's it! pH = -LOG[X], usually as base 10.
 
... one more thing. The "X" does not NEED to be CONCENTRATION, as you have plainly found.
pKa = -LOG(Ka)
 
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Well that's just a simple definition. I'm looking at how exactly pKa and pKb relate to everything else mathematically in the scheme of things.
 
Hi,
pA + pB =14. You can calculate the pH and the change of the pH during different stages of protolyses (?) with the help of the pA and say how a salt reacts with water. I hope I chosed the right vocabulary.
 
Last edited:
Firelion said:
I hope I chosed the right vocabulary.
Kind of...
 
Lancelot59 said:
Well that's just a simple definition. I'm looking at how exactly pKa and pKb relate to everything else mathematically in the scheme of things.

Sorry, but it sounds like "Ultimate Question of Life, the Universe, and Everything". And the answer is 42.

pKa and pKb are just ways of stating dissociation constant - they can be easily used to compare acid/base strength.

--
 
Thanks for the correction. I´m trying to get used to use English in everday life, so I will make errors , but I´m willing to learn ;)
 
Learning is fun!

@Borek: Ok, So the only thing I need to know is how to play around with them...well that's simple enough.

So pKa + Pkb = pKw?
 
  • #11
Lancelot59 said:
Learning is fun!

@Borek: Ok, So the only thing I need to know is how to play around with them...well that's simple enough.

So pKa + Pkb = pKw?

Yes, and that is why we use these not-so-silly "p" values. We have simple positive numbers making comparisons and some calculations easily performed.
 
  • #12
Awesome. I have another question.

I was reading through my textbook today, and it said that pKw is always equal to 14. Wouldn't it change with temperature? Or does it stay the same because temperature doesn't affect the auto-ionization of water?
 
  • #13
  • #14
:confused: Does anybody actually use pKbs?
 
  • #15
Usual route for calculation of pH of acetate (or more generally salts of weak acids) goes through pKb -> pOH -> pH.

--
methods
 
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  • #16
So that means that pKb = pOH? That's only at equivalance?
 
Last edited:

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