Polyprotic acids

[kiara chan]

Can someone please explain to me why when we are calculating the pH of polyprotic acids, we only use Ka1 and not the other Ka values?
THank you!

[Preston Pham 2H]

It depends on the reaction. Polyprotic acids tend to dissociate more than once such as H2So4. We would only use Ka1 to describe how it gets rid of a proton to become hs04. Another equilibrium constant would be used for another dissociation to happen.

[Akshat Katoch 2K]

Ka1 refers to the Acid dissociation of the first H+. Ka2 refers to the Acid dissociation of the second H+.
Example:
- H2SO4 - Ka1
- HSO4- - Ka2
Both acids will have a different acid dissociation constant because those are two different acids in a sense. H2SO4 is neutral and is more ready to give off an H+, but HSO4- is negatively charged and will cling to the H+ more resulting in different Ka values. You may have to use both Ka values if a question has a problem where both H+ get deprotonated.