# Homework Help: A Point Of Clarification With Acid/Base Ks

1. Oct 27, 2009

### Lancelot59

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?

2. Oct 27, 2009

### 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.

3. Oct 27, 2009

### symbolipoint

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

Last edited: Oct 27, 2009
4. Oct 27, 2009

### Lancelot59

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.

5. Oct 27, 2009

### 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.

Last edited: Oct 27, 2009
6. Oct 27, 2009

### Lancelot59

Kind of...

7. Oct 27, 2009

### Staff: Mentor

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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8. Oct 27, 2009

### 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 ;)

9. Oct 27, 2009

### 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?

10. Oct 28, 2009

### Staff: Mentor

11. Oct 28, 2009

### symbolipoint

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. Oct 28, 2009

### 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?

13. Oct 29, 2009

### Staff: Mentor

14. Oct 29, 2009

### epenguin

Does anybody actually use pKbs?

15. Oct 29, 2009

### Staff: Mentor

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

Last edited: Oct 29, 2009
16. Oct 29, 2009

### Lancelot59

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

Last edited: Oct 29, 2009