BTW, an exam question would likely not require you to solve a cubic equation, so if you have to, your instructor may expect you to use the 5% rule, possibly even if it is invalid to do so (when x or 2x or whatever is greater than 5% of the initial value). However, it is common to have to use the quadratic formula to solve quadratic equations for x. But again, you may be able to use the approximation, especially for weak acid and weak base equilibria. Steve |
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