### 5% rule

Posted:

**Sun Jan 20, 2019 10:10 pm**How does the 5% rule work? How do we know it should be used when calculating an equilibrium?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=51&t=40358

Page **1** of **1**

Posted: **Sun Jan 20, 2019 10:10 pm**

How does the 5% rule work? How do we know it should be used when calculating an equilibrium?

Posted: **Sun Jan 20, 2019 10:19 pm**

This is when approximating x while using the ICE table. For instance, if the equation becomes (x^2)/(.10-x)=K then you can approximate it to (x^2)/.10=K so you have to do easier calculations.

Posted: **Sun Jan 20, 2019 10:25 pm**

Additionally, if X is less than 5% of the initial concentration then the approximation made by eliminating X will be valid.

Posted: **Sun Jan 20, 2019 10:25 pm**

If the error of your approximation is less than 5% then using the approximation is fine. So you find your x value through the approximation method then divide by your initial amount of weak acid or base and multiply by 100. If the number calculated is greater than 5 then the quadratic formula should be used to solve for x. (x/[HA]) x 100 = some percent. Keep in mind this is the same formula for finding percent ionization of a weak acid or base.

Posted: **Sun Jan 20, 2019 10:34 pm**

Keep in mind that when approximating, Professor Lavelle said that it should be less than 10^-3 values. The 5% rule is simply a mechanism to check whether or not your approximations are valid.

Posted: **Mon Jan 21, 2019 3:00 pm**

When dealing with weak acids or bases, the 5% rule can be applied to neglect the change of the reactants towards the products in order to make calculating the change significantly easier without use of the quadratic formula.

Posted: **Mon Jan 21, 2019 4:35 pm**

When calculating the concentration of a reactant or product and given that Kc is less than 10^-3, you can disregard the x if it's something like "0.50-x" and approximate it to 0.50 only. When you get your final result, you then use the 5% rule to see if it is a valid approximation.

Posted: **Mon Jan 21, 2019 5:03 pm**

The 5% rule is useful because it allows us to simplify the equilibrium expression and avoid using the quadratic formula.

Posted: **Mon Jan 21, 2019 5:07 pm**

If the % pronotation is less than 5, then approximation (disregarding the initial change of x in the denominator) is valid.

Posted: **Mon Jan 21, 2019 8:27 pm**

If the percent protonation is <5%, then the approximation is valid and allows for finding x without using the quadratic formula

Posted: **Tue Jan 22, 2019 10:55 am**

If the K value is greater than 10^-3, that is when we cannot disregard x and have to solve using quadratic equation

Posted: **Wed Jan 23, 2019 9:53 am**

The 5% rule is just used to check if your approximation is valid. You can assume that you are allowed to approximate when the k value is <10^-3.

Posted: **Sun Jan 12, 2020 9:35 pm**

The five percent rule allows you to get out of using the quadratic formula when doing ice tables. Be sure to check if the x value you get is smaller than 5% of the initial value to get out of using the quadratic formula