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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).
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.
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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