CHEMISTRY COMMUNITY

What is the 5% rule and how do you apply it?

When calculating the percent ionization and it is less than 5%, it confirms that the approximation of x in the ICE table is okay and the use of the quadratic formula is not necessary.

because the change in x is so small, the original molarity of the molecule being changed by x is a suffice approximation.

If the percent deprotonation is less that 5%, then when we calculate equilibrium concentrations, we can ignore the x values relative to the initial concentration because it is not significant.

The 5% rule generally applies to approximation when trying to find x, after performing an ICE table. Instead of using the quadratic formula, you can disregard x relative to the initial if what you get from the approximation is less than 5% of the initial value. The procedure to finding that value is through the percent Ionization.

The 5% rule can be applied after making an approximation for x. You calculate x/[initial concentration given] and if the value is less than 5% (0.05) then the approximation is valid. However, if the value is greater than 5% then you must reject the approximation and recalculate x

it means you can only approximate a value for x on an ice table. you can sidestep the quadratic equation when you apply the approximation threshold. you do this because the value is so insignificant that it doesn't necessarily affect the solution

The 5% rule is used normally in indicating if when setting up the Kc expression if x should be factored into the concentration of the initial reactant. You normally approximate without using that x and if the resulting value x/(initial conc) is less than 5% it is valid to not factor that x in the concentration of the initial reactant.

The 5% rule is sort of like checking your work. If you approximated by ignoring x, if the percent ionization was less than 5%, then the approximation was fine.

The 5% rule is for checking whether or not your approximation is valid. This is usually if the K is less than 10^-3 and you take out the X from the denominator of your equilibrium expression.

The 5% Rule essentially means you can approximate the value for x on an ice table because the value is so small it doesn't make a significant change in the concentration. For example, Professor Lavelle talked about how a millionaire giving away $1,000 would barely notice the difference.