### 5 Percent Rule

Posted:

**Thu Dec 01, 2016 10:26 pm**I don't quite understand when to use the 5% rule, like when Dr. Lavelle says "If you lose $1000 from a $1,000,000 it won't make a noticeable difference."

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=66&t=17378

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Posted: **Thu Dec 01, 2016 10:26 pm**

I don't quite understand when to use the 5% rule, like when Dr. Lavelle says "If you lose $1000 from a $1,000,000 it won't make a noticeable difference."

Posted: **Thu Dec 01, 2016 11:03 pm**

I also agree to the fact I don't really understand the notation he made...

Posted: **Fri Dec 02, 2016 12:58 am**

He's implying that when you have an extremely large quantity of something, and if you take an almost negligible amount from it, you basically still have the same amount you started with. That's why for some of the acid/base problems you can assume that "X" is 0, because if it's such a small number and you subtract it from a large one, you will basically end up with the same large amount.

Posted: **Fri Dec 02, 2016 7:49 am**

I understand the fact that spending $100 is no different for a billionaire. But how does the 5% rule work in the context of solving x? How do we determine when to use the 5% rule? When do we use it????

Thanks,

Thanks,

Posted: **Fri Dec 02, 2016 10:45 am**

Generally you would find the solution first assuming that x is negligible (less than 5% of your initial concentration), then see if the answer is actually less than 5% of whatever concentration you started out with. If it is, then your solution is viable; if not, you would have to redo the question with the quadratic formula. It's basically a "shortcut" to solve the problem without having to use the quadratic equation.