### 5%

Posted:

**Thu Jan 24, 2019 10:16 pm**What is the 5% rule and how does it relate to the ICE box?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=49&t=40694

Page **1** of **1**

Posted: **Thu Jan 24, 2019 10:16 pm**

What is the 5% rule and how does it relate to the ICE box?

Posted: **Thu Jan 24, 2019 10:18 pm**

When you assume the change is small, you have to make sure that the answer you get is within 5% of the initial, so that the assumption is valid. Otherwise, you have to solve the quadratic equation.

Posted: **Thu Jan 24, 2019 11:11 pm**

So basically the "x" you calculate needs to be within 5% of the initial concentration so do x/initial x 100%

Posted: **Thu Jan 24, 2019 11:15 pm**

Your x has to be less than 5% of the initial concentration for your assumption/approximation to be valid

Posted: **Fri Jan 25, 2019 9:36 pm**

If you use the approximation method when using an ICE table, the x value you get needs to be less than 5% of the initial concentration for this method to be valid.

Posted: **Sun Jan 27, 2019 5:36 pm**

Is the 5% the measurement of protonation or deprotonation?

Posted: **Sun Jan 27, 2019 5:46 pm**

Yes, I agree. The 5% rule states that your amount has to be within this range of the initial concentration, if it is not you will have to solve for the variable using the quadratic formula and you would pick the positive x value thatâ€™s within the range.

Posted: **Sun Jan 27, 2019 5:53 pm**

If the x value is less than 5% of the initial concentration, then the approximation (of removing the -x from the equation) is valid.

Posted: **Sun Jan 27, 2019 5:54 pm**

If the concentration of H+ or OH- is less than 5% of the concentration of HA or HB, you can ignore the x when it is being subtracted or added(i.e. 2-x=2).

Posted: **Sun Jan 27, 2019 6:02 pm**

5% rule is a way you can test if you assume x is so small of a number it is negligible in finding x. for example if Kc= x^2 / (.55-x) you could try the assumption and calculate for x like this Kc=x^2 /(.55). Once you get a value you can test if it is less than 5% of the initial concentration given. If it is less then the assumption holds, if not you have to solve for x the quadratic way.

Posted: **Sun Jan 27, 2019 7:54 pm**

The 5% rule allows one to ignore the x in the denominator if the equilibrium constant is less than 10^-3 when performing ICE table calculations. The change in concentration (x) must be less than or equal to 5% of the initial concentration. If the change in concentration (x) is greater than 5%, then you cannot ignore the x and must solve a quadratic equation to find the change in concentration (x).