### X is to small

Posted:

**Sun Feb 09, 2020 9:24 pm**I read on chemistry community that the x is ignored if the K Is less than 10^-3 but is K always going to be given? If not how will I know when the x should be ignored?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=50&t=58812

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Posted: **Sun Feb 09, 2020 9:24 pm**

I read on chemistry community that the x is ignored if the K Is less than 10^-3 but is K always going to be given? If not how will I know when the x should be ignored?

Posted: **Sun Feb 09, 2020 9:29 pm**

I believe they should always give you K but I'm not completely sure. Also in the review session today, the K was smaller than 10^-3 but we still couldn't discard the x so I'm slightly confused.

Posted: **Sun Feb 09, 2020 9:30 pm**

If you didn't have K you wouldn't be able to solve for x. So I think we can assume that we either have to solve for K or for x but not both so in a scenario where you solve for x, K should be given.

Posted: **Sun Feb 09, 2020 9:34 pm**

I believe you can check to see if dropping the x is valid. You do the calculation (with x dropped) and then seeing if the resulting concentration value for x is less than 5 percent of the original concentration of solution (what you started with). If it is, then your approximation is valid.

Posted: **Sun Feb 09, 2020 11:31 pm**

X is dropped when it is less than 10^-3 and the percent at the end is less than 5

Posted: **Mon Feb 10, 2020 9:37 pm**

What values do you use to calculate if the percent at the end is less than 5.

Posted: **Mon Feb 10, 2020 9:41 pm**

You can disregard x when K is less than 10^(-3) because it will be an extremely small value that won't affect your calculations. However, to ensure that you were correct in disregarding the x value, you must check if x is less than 5% of the original solution's concentration.

Posted: **Mon Feb 10, 2020 9:53 pm**

When Kc< 10^-3 and x is less than 5% of the initial concentration then it is okay to approximate.

Posted: **Tue Feb 18, 2020 9:11 pm**

K will always be given when solving for x or else there would be no way for us to find x.