### 5%

Posted:

**Sun Jan 19, 2020 7:36 pm**What is the 5% deal that Lavelle talks about in equilibrium oriented questions. How do we get to that point?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=49&t=56701

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Posted: **Sun Jan 19, 2020 7:36 pm**

What is the 5% deal that Lavelle talks about in equilibrium oriented questions. How do we get to that point?

Posted: **Sun Jan 19, 2020 7:42 pm**

The 5% rule applies to when you use the ICE table. If K is less than 10^-3, then you can assume that X is so small that it will not make a difference. For example if you have K=X^2/(0.3-X), then you can change it to X^2/0.3 because the X in the denominator becomes negligible. The 5% rule is if you find that X is less than 5% of the initial concentration, then the approximation is valid.

Posted: **Sun Jan 19, 2020 7:45 pm**

If you approximate the result with a small Ka (<10^-3) and end up with a change value of <5% of initial value then your approximation is valid. Otherwise you need to do the full quadratic calculation.

Posted: **Sun Jan 19, 2020 8:02 pm**

When using approximations for ICE tables, if you find that X is less than 5% of the initial concentration, then the approximation is valid.

Posted: **Sun Jan 19, 2020 8:06 pm**

You're allowed to approximate X while doing ICE tables if the K constant is less than 1.0x10^-3. If the answer is less than 5% of the initial molarity, then the approximation is okay.