Sapling #2

[Emma Strassner 1J]

The Ka of a monoprotic weak acid is 0.00362. What is the percent ionization of a 0.142 M solution of this acid?

Hi, is this one of the instances where you can disregard the x in 0.142-x (from the ICE table) when working with the Ka equation?

[Sofia Azmat 1I]

Yes. You disregard it because in chem they approximate the answer to 0.142-x to be just about 0.142. This is because the value of x is so small that even if you were to keep x there it wouldn't matter because when you would use the appropriate amount of sig figs you would end up with 0.142 anyways.

[Zaid Bustami 1B]

So this is an instance where it's too close to call just by looking at your K_a. We know that our definition for a small K has an upper limit of 1x 10^-3, and since your given K value is really close it's better to stick with a more precise method of judging wither the assumption is valid. To save time and energy I would just use the quadratic equation anyways if you have a K value that's close to 1x10^-3, but technically you could use the small x assumption given that your x value is less than 5% of the initial concentration of the weak acid.
If you do your ICE table and solve for x by assuming 0.142 - x ≈ 0.142, you get x = 0.0227. Since 0.0227/0.142 = 16%, your assumption has too large of an error margin and as a result using the small x approximation is not valid here.
Like I said above, for values close to the threshold you could save yourself the effort of doing all this only to see that your approximation isn't valid, and instead just do the quadratic equation to be safe.

[Olivia Yang 3J]

You can assume the change in x is negligible if the initial concentration is more than 1000 times greater than the Ka. You can also verify this assumption by checking if the x is less than 5% of the concentration later.

[Emma Strassner 1J]

That makes sense, thank you!

[Daniela Santana 2L]

Hi! In order to check if this is an instance where you can approximate you can multiply the initial concentration by 1000. If the value you get is greater than Ka, you can approximate and disregard the x.

[Isaias Gomez D3A]

Yes, you can disregard. Another way is to work through and compare the concentrations at the end to check how close the % concentrations are.

[Geethika Janga 1L]

I don't think this would be one of the times we would disregard the x in 0.142-x because the value of Ka is not small enough to confidently get rid of the x. I think Professor said that we could safely disregard the value of x if the value of Ka is less that 10^-4. It would be safer just to do the quadratic equation. If you do decide to disregard the x, then you can check if that was okay with the % ionization, if this value is greater than 5% then you would have to use the quadratic formula.