### 5% Rule

Posted:

**Sat Mar 14, 2020 5:37 pm**Can someone elaborate what the 5% rule is?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=123&t=62502

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Posted: **Sat Mar 14, 2020 5:37 pm**

Can someone elaborate what the 5% rule is?

Posted: **Sat Mar 14, 2020 5:38 pm**

when using the approximation (removing x when you are subtracting/adding it to something) you must check to make sure that x is less than 5% of the reactant that it is formed from (x will likely be H or OH- so you would use the acid or base)

Posted: **Sat Mar 14, 2020 5:40 pm**

So if it meets the rule, do we just ignore the value?

Posted: **Sat Mar 14, 2020 5:41 pm**

You put the concentration you got from approximated the concentrations in the equilibrium values (from an ICE table) over the original concentration that's given and if it is less than 5% the approximation is sound, then if not then you have to use the quadratic formula to find the values.

Posted: **Sat Mar 14, 2020 5:43 pm**

You also know that you can use the approximation if your equilibrium constant, K, is small enough.

Posted: **Sat Mar 14, 2020 5:58 pm**

The five percent rule is used in ICE tables. If the x value you get is 5% or less than the initial then you can ignore it. I believe that if the K is 10^-3 or less you can usually make this assumption.

Posted: **Sat Mar 14, 2020 7:31 pm**

Its for simplifying ICE calculations. If the end result is less than 5%, than the approximation is accurate.