pH calculations for weak acids

[stuw]

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Just wanted to clear something up...

So say we were given H2S (with some given Ka value), and we are asked to find the [H+].

From my understanding we break write the equation like this:

H2S <--> 2[H+] + [S^(2-)]

So when writing out the Ka equation...:

Ka = {([H]^2)([S^(2-)]} / [H2S]

My question is...will [H]=2x or simply just "x"? I'm guessing it'll be 2x since we're looking at molar concentration here, right? This is the way we solve Ksp problems as well.

So...

Ka = {([2x]^2)(x)}/[H2S] ?

Is that the best/correct way to approach a problem like this?

 

[Maxxxx]

H2S would be diprotic. The first pKa is only for H2S -> [H+] + [HS-]. A second pKa is used for [HS-] -> [H+] + [S-2]. And I'm pretty sure pKa2 is be very big

 

[stuw]

So it will almost always be for one concentration of [x]? Also, why is it that the second pka will be larger? Thanks again guys