5% Rule

[Sasha Gladkikh 2A]

What is the 5% rule? Can this rule be applied to any chemical equilibrium problem that involves an ICE table and approximation while solving?

[Bryan Cheng 1H]

The 5% rule is simply that we can consider a change in initial quantities to equilibrium negligible (and therefore use an approximation) if the change in a reactant species using the approximation is less than 5%. This is a semi-arbitrary guideline, but it is most useful in acid/base equilibrium. Generally, it can be applied to any equilibrium problem.

[Molly Smith 1J]

The 5% rule is a general guideline for whether or not your approximation was valid to make. In most cases, you should already have a pretty good idea if it was valid to make the approximation based on the K value. The 5% rule is just a way of double checking that you did approximate accurately.

[Matthew Vu 3C]

The 5% rule is used for when weak acids and bases dissociate so little that the change in initial concentration is practically negligible so that when we solve ICE boxes, we can get rid of the change in initial concentration (get rid of the x).

You can only use this rule when the acid dissociates less than 5%, hence the name. Generally, you can use it when Ka is very small, but a safe way you can check it is if you assume the 5% rule, and when you calculate the final concentration of [H30+] or [OH]-, it should be less than 5% of the initial concentration of the weak acid or base.

[Jenny Han 3F]

Additionally, the approximation by way of "ignoring" the x or change in concentration in the denominator is limited to that particular x only (or the ones in the denominator) since equating any x in the numerator to 0 would render the whole fraction 0 and the problem unsolvable.

[Eszter Kovacs 1A]

when less than 5% of the given substance dissociates the x value can be ignored, because its value is so insignificant.

[Anne Hogue 2J]

The 5% rule tests whether you can use your approximation. If the percent is more than 5% then you must redo the problem with the given small number. If it is less than 5%, then the approximation is valid.

[AChassion Dis2A]

From my understanding the 5% rule occurs when checking your answer in regards to the percent ionization of a weak acid/base. Because the Ka or Kb is so small for these acids or bases, we sometimes assume x is essentially negligible to avoid going through a messy quadratic.
At the end, our value for percent ionization should be less than 5 percent. If it is greater, then we shouldn't have assumed Ka or Kb was small enough to make x negligible in our quadratic equation.

[Stephanie Rodas 2C]

Usually Ka and Kb values are within the range of 5% of the actual answer. If your calculation is more than 5%, then you cannot drop the minus x when you continue with the calculation. Without doing this, it wouldn't really be possible to calculate the x.