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When is X negligible.

Posted: Fri Jan 18, 2019 3:21 pm
by Samantha Jimenez 4k
So I'm still pretty confused as to when we should know when x can be disregarded in the denominator for the quadratic equation in an ICE problem. Is it safer to just never disregard it, or are there exceptions when it is acceptable?

Re: When is X negligible.

Posted: Fri Jan 18, 2019 3:24 pm
by madisondesilva1c
X is negligible if the K value is below 10^-3. Therefore it would not change the initial concentration of the given reactant, so it is able to be disregarded in these cases.

Re: When is X negligible.

Posted: Fri Jan 18, 2019 3:24 pm
by Shubham Rai 2C
Usually, it depends on the problem. You can ignore the X when the K value is smaller than 10^-3. However, for the numerator, it would be best to keep the x as usually, that's the x value that is being solved for.

Re: When is X negligible.

Posted: Fri Jan 18, 2019 3:59 pm
by Ariel Cheng 2I
As the others mentioned before, the general rule is that you can disregard the X when the K value is smaller than 10^-3. Afterward, you can also confirm that the X you get is less than 5% of the initial concentration in order to double check that your approximation is valid.

Re: When is X negligible.

Posted: Fri Jan 18, 2019 4:33 pm
by JadeSebti1L
My TA also said today it is not mandatory to do the shortcut and that the quadratic works fine. But, if you have to do the shortcut because of time constraints you have to check that x is less than 5% of the initial concentration to get credit for that step.

Re: When is X negligible.

Posted: Sat Jan 19, 2019 12:41 pm
by KHuang1L
X is negligible when K is less than 10^3. Also confirm that your approximation is valid if the X value is less than 5% of your initial concentration.

Re: When is X negligible.

Posted: Sat Jan 19, 2019 3:58 pm
by Aleeque Marselian 1A
You can approximate the x in the denominator when the value of K is less than 10^-3. However, it is not mandatory and you would still get the same value of x if you choose not do use the shortcut.