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### 5 percent rule

Posted: Thu Jan 16, 2020 10:00 pm
In class, Lavelle mentioned something called the 5 % rule. I didn't really understand this concept. Can someone explain when to use this? Does it only apply to acids and bases ICE calculations?

### Re: 5 percent rule

Posted: Thu Jan 16, 2020 10:05 pm
The 5% rule refers to the acceptable percentage of x in relation to the initial amount that makes it okay to approximate the percent ionization with weak acid and base calculations.

### Re: 5 percent rule

Posted: Thu Jan 16, 2020 10:07 pm
If the percent deprotonization or percent protonization is less than 5%, then the x value is valid. If it is not, then the quadratic formula is required and an estimate is not a safe approximation for the reaction.

### Re: 5 percent rule

Posted: Thu Jan 16, 2020 10:42 pm
The percent protonization or deprotonization needs to be less than 5% for the x value to be valid; it makes it acceptable to use approximations.

### Re: 5 percent rule

Posted: Fri Jan 17, 2020 3:33 pm
The 5% rule should be considered when you remove x- additions/subtracts to make the ICE equation easier to solve (when K <10^-3)

### Re: 5 percent rule

Posted: Fri Jan 17, 2020 3:37 pm
Divide the x value that you find by the initial concentration to check if it is less than 5% of the initial concentration (less than 5% is deprotonated). This checks if it is small enough to disregard if being subtracted from the initial concentration, making your calculations much easier.

### Re: 5 percent rule

Posted: Fri Jan 17, 2020 3:45 pm
The 5% rule is just a way of making sure that when you remove the x from the bottom of the fraction, you aren't affecting the answer. The idea is if x is less than 5% of the number its being subtracted from, it makes such a little difference that it is mathematically ok to assume x is 0 relative to the bigger number.

### Re: 5 percent rule

Posted: Fri Jan 17, 2020 4:05 pm
What approximation is it talking about?

### Re: 5 percent rule

Posted: Fri Jan 17, 2020 4:34 pm
You can use the approximation when your K value is smaller than 10^-3 so you can save time not doing the quadratic equation. This is because a value that small in comparison to the initial values given will not affect your x value by much. After solving for x, you can do the % depronated. This is calculated by the x value over the initial value, it should be less than 5% to know that the approximation was valid.

### Re: 5 percent rule

Posted: Fri Jan 17, 2020 8:33 pm
The 5% rule is used to verify that assuming x to be negligibly small is a correct assertion. It is simply the calculation of percent protonation.

### Re: 5 percent rule

Posted: Sat Jan 18, 2020 2:10 pm
You use the 5 percent rule to make sure that the approximation that you made where x is 0 in the denominator when k is less than 10^-3 is valid and should be used every time you solve for equilibrium concentrations using the ICE chart.