Welcome Guest Search | Active Topics |

Tag as favorite
ICE 149: using pKa to find pKb
Options
Previous Topic Next Topic
Gilbert_5394
#1 Posted : Sunday, June 07, 2020 4:12:53 PM
Rank: Newbie

Groups: Registered
Joined: 6/7/2020
Posts: 5

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
For Q149, they found the pkb for HSO3- through the pKa of H2SO3. How come we cant use the given pKa of HSO3- and do pKa+pKb = 14?
Back to top
Report?
INSTR_Katerina_102
#2 Posted : Sunday, June 07, 2020 5:36:10 PM
Rank: Advanced Member

Groups: Registered
Joined: 6/24/2019
Posts: 250

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi,

The pKa and pKb adding to 14 works for very specific scenarios.

Imagine the case of

H2A --> HA-.--> A^2-

There are two dissociations here. The first can be defined as H2A --> HA-, which has a pKa1 for the forward reaction and a pKb1 which dictates the reverse reaction (HA- --> H2A). These values of pKa1 and pKb1 will add to 14. Note that you need to add the pKa for H2A and the pKb for HA-, because you need the conjugate acid and base.

However, you also have pKa2 for (HA --> A^2-), and pKb2 for the reverse reaction. You can add pKa2 and pKb2 together to get 14 here, where pKa2 is for HA- and pKb2 for A^2-.

Note that because pKa and pKb are defined for very specific processes, you have to use the right one for the situation (ie the conjugate acid base pair). That is, pKb2 + pKa1 =/= 14 and pKb1 + pKa2 =/= 14.

Extending this logic to your example, you use H2SO3's pKa because it is the conjugate acid pair to HSO3-.
Back to top
Report?
Users browsing this topic
Guest
Tag as favorite
You cannot post new topics in this forum.
You cannot reply to topics in this forum.
You cannot delete your posts in this forum.
You cannot edit your posts in this forum.
You cannot create polls in this forum.
You cannot vote in polls in this forum.

Clean Slate theme by Jaben Cargman (Tiny Gecko)
Powered by YAF | YAF © 2003-2009, Yet Another Forum.NET
This page was generated in 0.083 seconds.