5% rule

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Peter Nguyen 2I
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

5% rule

Postby Peter Nguyen 2I » Sun Jan 20, 2019 2:18 pm

Is it always safe to assume that any Ka that is less than 10^-3 will result in an X that is less than 5%?

Adrian C 1D
Posts: 60
Joined: Fri Sep 28, 2018 12:19 am

Re: 5% rule

Postby Adrian C 1D » Sun Jan 20, 2019 2:26 pm

It wouldn't hurt to expect that when you see that the Ka is that low, but I wouldn't count on it every single time. It would still be good to go through the entire process.

Posts: 87
Joined: Fri Sep 28, 2018 12:29 am

Re: 5% rule

Postby Mukil_Pari_2I » Sun Jan 20, 2019 3:10 pm

As Dr. Lavelle said in lecture, less than 5% is the rule. You can check with the quadratic formula to confirm though.

Albert Duong 4C
Posts: 66
Joined: Fri Sep 28, 2018 12:17 am
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Re: 5% rule

Postby Albert Duong 4C » Sun Jan 20, 2019 3:34 pm

Most often, the Ka will be way less than 10^-3 (on tests and such) so as to avoid confusion on whether to approximate or not in my experience.

caseygilles 1E
Posts: 73
Joined: Fri Sep 28, 2018 12:18 am

Re: 5% rule

Postby caseygilles 1E » Sun Jan 20, 2019 5:32 pm

Yes it is safe to assume this, although checking the percent ionization at the end is always smart just to make sure.
To do this use the formula:
% ionization= ((molarity of conjugate base formed ) /(initial molarity of acid)) * 100%.
If this number is <5% then we can use approximate for the change in concentration.

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