Percent Ionization for Weak Acids (Week 2 Homework)

[Sasha Gladkikh 2A]

For #2 on the "Week 2 Achieve Homework", the question states:

"The K_a of a monoprotic weak acid is 0.00215. What is the percent ionization of a 0.199 M solution of this acid?"

I calculated a percent ionization of 9.87% and it was correct; however, this percent is not less than 5%, yet it is a weak acid. Do all weak acids have to have a percent ionization of less than 5% (i.e., follow the 5% rule)?

[Mari Masunaga 1G]

Hi! I don't think that weak acids necessarily need to follow the 5% rule (although to be honest I could be wrong). Strong acids dissociate completely and so their percent ionization is 100%. I guess that as long as percent ionization is less than 100% is it not a strong acid (and therefore a weak acid?). To my understanding, we just use the 5% rule to determine if "x" needs to be included in ice box calculations. sorry this is not a super definitive answer but i hope this helped a little.

[Ishant Goel 2D]

I'm also not so sure because with using the approximation and not using the approximation, we still get above 5% for the percent ionization. I think if we just use the approximation and it comes over 5%, we stick to using the quadratic formula and if that comes over 5%, we just accept that number to be the percent ionization.

[Caroline 2A]

Hi!

We can check the 5% rule by dividing the value of X by the initial concentration and multiplying that value by 100%. If x is less than 5% of the concentration, then our assumption is valid (and it is valid in this case).

Hope this helps :)

[Maggie_Harned_3H]

Hi! I don't think that all weak acids have to have a percent ionization under 5% to be considered weak because percentage ionization for a solution is dependent on the concentration of the solution. So for example the smaller the concentration of solution the higher the percentage ionization is, so if we were to look at a different concentration of a weak acid in a solution we would see that this ratio of initial concentration gives us a low percent ionization showing its a weak acid.