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### 5% vs. K < 10^-3

Posted: **Sun Jan 19, 2020 1:24 pm**

by **Jared Khoo 1G**

What is the difference between the 5% rule and the rule saying that you can disregard x in the denominator if K < 10^{-3}, or are they the same thing. Also, is this mentioned in the textbook and if so, where?

### Re: 5% vs. K < 10^-3

Posted: **Sun Jan 19, 2020 1:41 pm**

by **Deepika Reddy 1A**

They are the same rule. If you disregard the x because K is less than 10^-3, at the end you can also check if it corresponds to the 5% rule. If it does follow the 5% rule, this means that the substance barely dissociates so K is small so it was correct to disregard x.

### Re: 5% vs. K < 10^-3

Posted: **Sun Jan 19, 2020 4:02 pm**

by **Emily Lo 1J**

They're the same. You can disregard the X in the denominator when K < 10^-3 which essentially allows you to solve for X without the quadratic formula. It also means that X is small enough to be excluded in the equation.

### Re: 5% vs. K < 10^-3

Posted: **Sun Jan 19, 2020 9:17 pm**

by **AniP_2D**

When K<10^-3, you are allowed to approximate and the 5% rule is just a means of confirming that your approximation is valid.