Finding Predominant Species Present

[Cassidy Sadowski 3E]

Hi! I'm confused surrounding questions 9 & 10 of Assignment 2. Attached is a picture of question 9 for reference. Based on the pH what steps should I take to identify the predominant species?

[Nathan Chu 2G]

Hello,

Dr. Lavelle posted a document on his main website going over predominant species. The homework problem's solution reasons the "Chem 14A way." In Chem 14B, we use an equation (which we didn't learn) called Henderson-Hasselbach:

pH = pKa + log([A-]/[HA]) and pOH = pKb + log([HB+]/[B])

For problem nine, input the given pH and pKa value into the above equation. Subtract pKa from both sides and you would get (some value)=log([A-]/[HA]). Get rid of the logarithm by doing 10^(some value) = 10^log([A-]/[HA]). After finishing this step, you now have a ratio. If 10^(some value) is small, then that must mean the denominator ([HA]) is predominant. If 10^(some value) is large, then that must mean the numerator ([A-]) is predominant. The acidity is determined by what the predominant species turns out to be; if HA is predominant, the solution will be acidic as there will be a lot of the acid. If B is predominant, the solution will be basic as there is a lot of the conjugate base.

Similar logic is true for problem 10.

Hope this helps. Cheers!