Why are there multiple Ka values

[Laila Alkhouri 3H]

I was looking at table 6E.1 and I was wondering why there are multiple Ka and pKa values. What does this mean and how do we know which ones to use?

[Stephanie_manasyan]

Hi! There are multiple listed on the table because the Ka1 is the value for the first protonation and Ka2 is the value for the second protonation. Unless a problem directly asks us to use the second protonation, we always use the first one (so Ka1) Hope this helped!

[Elena Liu 2J]

There's multiple Ka values because the table is for polyprotic acids, acids that give more than 1 H+. Therefore, it will go through multiple reactions to give out all the H+. For example, for H3PO3 will react water to form H30+ and H2PO3-, for which Ka1 corresponds to. Then, the H2PO3(-) formed will react with water to form H30+ and HPO3(2-), for which the Ka2 corresponds to.