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