## 5 %

305385703
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### 5 %

Why is the 5% rule a thing? It seems a bit arbitrary to neglect that much of a change in the amount of reactants. Is there any practical basis in the rule.

Jonathan Gong 2H
Posts: 105
Joined: Sat Jul 20, 2019 12:16 am

### Re: 5 %

After searching this up, the rule it exists because values of Ka and Kb are typically known only to an accuracy of +- 5%. Thus, if the approximation is within 5%, we can consider it valid.

Posts: 111
Joined: Thu Jul 25, 2019 12:17 am

### Re: 5 %

I was doing a homework problem and I was using my calculator's quadratic function to solve the Kc equations. The funny part, I guess, is that my x value was so small that when I subtracted it from the initial value the first two digits/numbers were still the same. When you have two (or three) significant figures in the problem, sometime you can't even show the difference.

Also, I imagine that since we are only allowed a scientific calculator, and "practicality purposes," chemists just use the 5% rule in labs etc. because it makes the problem significantly easier to solve.

Shutong Hou_1F
Posts: 117
Joined: Sat Sep 14, 2019 12:17 am

### Re: 5 %

Exactly! And also if the percentage ionization of a substance is less than 5%, then the percent of the substance that is ionized is relatively small compared to the initial concentration of the substance, so we could disregard the decrease in the amount of the substance when mentioning the substance's current concentration in calculation.

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### Re: 5 %

If you solve Ka or Kb by the approximate method and exceed 5% in order to obtain as accurate an answer as possible, the quadratic method must be used. So I guess, it's just a method of making finding the solution simpler instead of going through the whole quadratic formula if it isn't necessary.

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