Sapling Week 2 Question 3

[Kaylee Messick 3J]

For this question, the Ka = 8.40x10^-4. When solving this problem, I decided to use approximation, even though the Ka was at 10^-4. I got a percent ionization that was larger than 5%, so I went back to solve the problem with the quadratic formula. I got essentially the same answer as my approximation. Would this be an exception to making sure x is less than 5% when using approximations or are there other factors that affect the percent ionization?

[Kaylee Nezwek 1D]

hey! for this question I don't think you can use an approximation because the 8 in 0.0008 would technically make it round to 0.001, which is greater than 10^-4, the cut off discussed in lecture. Instead, I used the quadratic formula to get an x of 0.0092, and then for the percent ionization I divided this number by the M given in the problem. I'm not sure if this answers your question completely but hope it helps!

[Alison Perkins 2B]

rather than approximating, make an ICE table, and use the Ka value and equation to solve for your x value.
Then, take [H+]/[HA] at equilibrium x 100% for percent ionization.

[EmilyGillen_1A]

I did the same thing when I first did the problem, but Kaylee is right, your value has to be less than 1.0 * 10^-4 for the method to work. So if the first number is greater than 1.0 it will not be small enough to ignore in your ICE box final equation. I find that just for my own thought process, I only use this method if its =10^-5 or less. But you can always check the percent ionization to see if there is something you made a mistake on.

[Mari Williams 1K]

I did both an approximation and without it, but got almost the same answer. Sapling only accepts the non-approximated with like 4 decimal places

[Juliana Rosales 1H]

I had a similar issue! I have the same problem as you, but I think I will try it with the approximation and without. I know the professor said something about a cutoff at 10 to the 4th, but I don't know if that applies to sapling as well.

[uclakat]

this question was hard! thanks for the help