Question on 12.41

[Joanna Pham - 2D]

For this question, it says to use tables 12.1 and 12.2 to place the bases in order of increasing strength: F-, NH3, CH3CO2-, and C5H5N.

Only C5H5N is present on the tables. How do I determine which base is stronger and weaker (F-, NH3, and CH3CO2-)?

[Charles Hood Disc 1C]

The others are acids, not bases so they can be found in table 12.1, except bonded to another element (generally hydrogen) this gives you the K_a value instead of the K_b value. This means that you can can subtract the pK_a from 14 to find the pK_b.

[Ethan Baurle 1A]

Also, in regards to the strengths of acids and bases based on their equilibrium constants, a lower Kx (Ka or Kb) shows a weaker substance. A lower Kx also responds to a higher PKx if those are the values you are looking at, so a higher PKx would be weaker.

[A De Castro 14B 2H]

I believe both C₅H₅N and NH₃ are in table 12.2. As for the anions, you can find their pK_b values by subtracting the pK_a values of their parent acids (HF and CH₃COOH), found in table 12.1, from 14 (which is pK_w). The lower the pK_b value, the stronger the base is.