Making the Approximating for X: K or % Ionization?

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Ayesha Aslam-Mir 3C
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Making the Approximating for X: K or % Ionization?

Postby Ayesha Aslam-Mir 3C » Sun Jan 24, 2021 8:13 pm

I was wondering which is a more reliable indicator for maknig the approximation? There's a question on the Sapling homework where the K is very small (<10 to the -3) but the % ionization is greater than 5%. In a case like this, which indicator is better?

Vivian_Le_1L
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Re: Making the Approximating for X: K or % Ionization?

Postby Vivian_Le_1L » Sun Jan 24, 2021 8:20 pm

It's better to check with % ionization. If the % ionization is greater than 5% then you can not approximate even if your K is less than 10^-3.

Evelyn Silva 3J
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Re: Making the Approximating for X: K or % Ionization?

Postby Evelyn Silva 3J » Sun Jan 24, 2021 8:21 pm

If Ka is close to 10^-3 then I always check the percent ionization because sometimes it is greater than 5% and in that case, we can't assume the change is neglible. But if Ka is extremely small like 10^-12 then you can automatically ignore the x.

Ivan Chen 2H
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Re: Making the Approximating for X: K or % Ionization?

Postby Ivan Chen 2H » Sun Jan 24, 2021 8:22 pm

I thought a small K value like less than 10^-3 was more of helpful indication that x might be negligible, and the 5% ionization was more of a definite indication that it was actually negligible.

Kaylee Messick 3J
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Re: Making the Approximating for X: K or % Ionization?

Postby Kaylee Messick 3J » Sun Jan 24, 2021 8:29 pm

I would think that the percent ionization is most likely more helpful as an indicator because as seen in the Sapling question, sometimes the approximation using the value of K doesn't always work, so it is safer to check to make sure it is less than 5%. Hope this helps!

Kelly Tran 1J
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Re: Making the Approximating for X: K or % Ionization?

Postby Kelly Tran 1J » Sun Jan 24, 2021 9:12 pm

Hi! If K < 10^-4, then you can make your approximation of x. However, if you are unsure about whether or not you can make the approximation, you can calculate the % ionization to determine whether the approximation is valid. So, if (x/initial concentration) x 100 < 5%, your approximation is valid. If it is greater than 5%, you would not be able to make the approximation. Hope this helps!

Sondia Luong 1C
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Re: Making the Approximating for X: K or % Ionization?

Postby Sondia Luong 1C » Sun Jan 24, 2021 9:19 pm

The surest way to check whether or not you can approximate for x is by using % ionization. If the percent ionization is less than 5%, your approximation will work! I would check percent ionization when K is around 10^-4 because it's close to 10^-3, which is the cutoff value of when you should use x approximations.


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