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You can use approximation for weak acids and bases problems when the K value is less than 10^-3. This means it is small enough to not make a huge difference when calculating concentrations, so you wouldn't need to use the quadratic formula. The 5% rule refers to evaluating whether the approximation was valid: getting a percent ionization of less than 5% means the approximation is good.
Approximation can be used for weak acids and bases problems when the K value is less than 10^-3. This means it is so small that it is not going to make a difference when calculating concentrations. After you use approximation, you should use the 5% rule to check if your calculation was valid/ if the change of concentration was really small enough to consider negligible. The 5% rule states that if the percent ionization is less than 5%, it is okay to use approximation.
If the K value is less than 10^-3, then the concentrations of products created are small enough in comparison to the original concentration of the reactant(s) that you can consider it insignificant enough to leave the “-x” term out of the reactants when calculating K.
ashwathinair wrote:Can someone explain how and when we use approximation for ICE tables? Are there other situations you can use them as well? Is this only for acids and bases?
If the K value is less than 10^-3, then for the creation of the equation that is equivalent to the equilibrium constant, it is acceptable to remove the -X value from the equilibrium constant equation. This is because if the equilibrium constant is small, then the concentration of products is small, thus the concentration of reacts remain relatively large, so any change of X is basically nothing (but IT IS NOT 0).
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