# Homework Help: Just brushing up on chemistry

1. Nov 28, 2007

### lostsheep

Hey guys, im new here. I am a college student that is trying to brush up on some chemistry. I am reading an old textbook that I have, and I didnt understand something.

I was wondering how some would use the Kb of a weak base, how they would determine the value of Ka for the conjugate base?

A sample problem in the book is: A weak acid has a Kb= 1.5 x 10^-9. What is the value of Ka for the conjugate base?

I have read the whole chapter over Acid & Bases, but it says this problem is on the cd-rom that is included with my book. Yet, i never recieved that cd when I bought my book. I have tried searching but have had no luck. Any help would be great. I am just trying to understand the relationship between Ka and Kb. Thanks again.

2. Nov 28, 2007

### eli64

Kw = Ka x Kb = 1 x 10^-14

3. Nov 28, 2007

### ShawnD

K is called a dissociation constant

What K states is that there is a certain amount of dissociation at any given time, and almost everything has one of these numbers. Something like lead chloride will have a K value to say how soluble it is. Here you are talking about water, and K relates to water breaking from HOH into H and OH. For water, the dissociation constant is 10^-14.

$$[H^+] * [OH^-] = 10^{-14}$$

$$K_A * K_B = 10^{-14}$$

$$pK_A + pK_B = 14$$