## 5% Rule

gabbym
Posts: 76
Joined: Wed Apr 11, 2018 3:00 am

### 5% Rule

Hey guys, I'm still a little confused on the 5% Rule. How do you know when to get rid of the x when you have something like x^2/0.20-x?

Becky Belisle 1A
Posts: 81
Joined: Fri Sep 28, 2018 12:18 am

### Re: 5% Rule

If you are given the Ka and it is very very small then sometimes it can be assumed that less than 5% of the acid was deprotonated. I think it also has to do with memorizing which acids and bases are very weak.

dgerges 4H
Posts: 65
Joined: Fri Sep 28, 2018 12:24 am

### Re: 5% Rule

you can know it's okay to use it if the K is below 10^-3!

Kavvya Gupta 1H
Posts: 35
Joined: Fri Sep 28, 2018 12:21 am

### Re: 5% Rule

if the Ka value is below 10^-3 then it is okay to assume that you can get rid of the -x when solving for x because you assume that the change is less than 5%. After solving for x though, just make sure to check that, in fact, it is less than 5% by using the equation x/(acid)initial * 100