[jamesmustain]

Hi, I know the values of my Ka1 and Ka2 for a diprotic acid. How could I find the Ka value? I can't really find anything online or in my textbook. Is it as easy as adding the two numbers? Thanks for help?

[Meter]

There isn't a single Ka value for a diprotic acid as the Ka value is an expression for the dissociation of one proton into solution.

Think about it like this: [tex]
K_{a1} = \frac{[H_3O^+][HA^-]}{[H_2A]}
[/tex]
and [tex]
K_{a2} = \frac{[H_3O^+][A^-]}{[HA]}
[/tex]
Now, usually [tex]K_{a1} >> K_{a2}[/tex], meaning that the first proton of a diprotic acid usually dissociates more easily than the second proton, so you could say that - in general - that K__a1 is the best metric for the strength of your acid. For example, H₂SO₄ has a pKa1 = -2.89 and its mono-deprotonated version (bisulfate ion, HSO₄^-) has pKa2 = 1.99.

[Borek]

Ka1 and Ka2 are so called stepwise constants, there is also an overall constant:

[tex]K_{a1,2} = \frac {[H^+]^2[A^{2-}]}{[H_2A]} = K_{a1}*K_{a2}[/tex]

But it still doesn't count as "just Ka" and it alone is not enough to calculate pH of a diprotic acid solution.

[Meter]

Ka1 and Ka2 are so called stepwise constants, there is also an overall constant:

[tex]K_{a1,2} = \frac {[H^+]^2[A^{2-}]}{[H_2A]} = K_{a1}*K_{a2}[/tex]

But it still doesn't count as "just Ka" and it alone is not enough to calculate pH of a diprotic acid solution.

When and where is this applied? I can't readily think of an example.

[Borek]

For example:

https://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base

In general you can define dissociation constants in many ways, some of them are more convenient to use in some contexts.