A Point Of Clarification With Acid/Base Ks

[Lancelot59]

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

K_W=(K_a)(K_b)

K_w being the ionization constant of water, with K_a,and K_b being the ionization constants for acids and bases respectively.

Then there's pK_W = pH + pOH

However I get confused when I get to pK_a and pK_b. 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 pK_a and pK_b relate back to everything else? Also could I get a better definition of pH/pOH, or is my definition good enough?

 

[symbolipoint]

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.

 

[symbolipoint]

... one more thing. The "X" does not NEED to be CONCENTRATION, as you have plainly found.
pKa = -LOG(K_a)

 

[Lancelot59]

Well that's just a simple definition. I'm looking at how exactly pK_a and pK_b relate to everything else mathematically in the scheme of things.

 

[Firelion]

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.

 

[Lancelot59]

I hope I chosed the right vocabulary.

Kind of...

 

[Borek]

Well that's just a simple definition. I'm looking at how exactly pK_a and pK_b 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.

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[Firelion]

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 ;)

 

[Lancelot59]

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?

 

[Borek]

So pKa + Pkb = pKw?

Yes, see Brønsted-Lowry's acids and bases.

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[symbolipoint]

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.

 

[Lancelot59]

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?

 

[Borek]

It is not always 14. See water ion product.

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[epenguin]

Does anybody actually use pK_bs?

 

[Borek]

Usual route for calculation of pH of acetate (or more generally salts of weak acids) goes through pKb -> pOH -> pH.

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methods

 

[Lancelot59]

So that means that pKb = pOH? That's only at equivalance?