Making the Approximating for X: K or % Ionization?

[Ayesha Aslam-Mir 3C]

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]

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]

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]

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]

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]

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]

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.