5% rule

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Samuel G Rivera - Discussion 4I
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Joined: Sat Aug 17, 2019 12:16 am
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5% rule

Postby Samuel G Rivera - Discussion 4I » Sun Jan 19, 2020 6:45 pm

I keep hearing the 5% rule, but I'm still confused about what it is and when to use it.

Rodrigo2J
Posts: 102
Joined: Sat Jul 20, 2019 12:16 am

Re: 5% rule

Postby Rodrigo2J » Sun Jan 19, 2020 6:50 pm

Basically, the 5% rule can be used when the equilibrium constant is really small (less than 10^-3). It allows you to assume x on the denominator is negligible. So if we had Ka=(X^2)/(.5-x) and Ka was equal to 5*10^-8, then we could pretend that the x in the denominator isn’t there. So the equation would become 5*10^-8=(x^2)/(.5).

Christine Honda 2I
Posts: 116
Joined: Sat Sep 14, 2019 12:17 am

Re: 5% rule

Postby Christine Honda 2I » Sun Jan 19, 2020 6:51 pm

The 5% percent rule is used to check if our "X is small" approximation is valid or not. We check to see if the X we solve for is less than 5% of the initial concentration. If it IS less than 5% then the approximation is valid.

KarineKim2L
Posts: 100
Joined: Fri Aug 30, 2019 12:16 am

Re: 5% rule

Postby KarineKim2L » Sun Jan 19, 2020 6:51 pm

The 5% rule applies to when you use the ICE table. After you have your K in the form of Xs, if K is less than 10^-3, then you can assume that X is so small that it will not make a difference. So for example, if you have K=X^2/(0.15 - X), then you can change it to X^2/0.15, as the X in the denominator will not alter the calculations very much. Then, you check that your approximation is valid using the 5% rule. If X that you found is less than 5% of the initial concentration, then the approximation is valid.


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