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If the error of your approximation is less than 5% then using the approximation is fine. So you find your x value through the approximation method then divide by your initial amount of weak acid or base and multiply by 100. If the number calculated is greater than 5 then the quadratic formula should be used to solve for x. (x/[HA]) x 100 = some percent. Keep in mind this is the same formula for finding percent ionization of a weak acid or base.
When dealing with weak acids or bases, the 5% rule can be applied to neglect the change of the reactants towards the products in order to make calculating the change significantly easier without use of the quadratic formula.
When calculating the concentration of a reactant or product and given that Kc is less than 10^-3, you can disregard the x if it's something like "0.50-x" and approximate it to 0.50 only. When you get your final result, you then use the 5% rule to see if it is a valid approximation.
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