# The acid dissociation constant of #"H"_2"S"# and #"HS"^-# are #10^-7# and #10^-13# respectively. The pH of 0.1 M aqueous solution of #"H"_2"S"# will be?

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##### 1 Answer

Disclaimer: Somewhat long answer, but the answer is not as bad as one might think!

#### Explanation:

To find the

Let's set up some equation using the

This acid will dissociate in two steps. We are given the concentration of

Then we can assume that both of these species are in a 1:1 ratio in the dissociation, allowing us to take the square root to find the concentration of both species:

Now in the second dissociation,

Same principle to find the concentration of

Hence:

So the combined concentration of

So the second dissocation was so small it did not really impact the pH. I guess if this was a multiple choice exam then you only needed to look at the first dissociation and find the square root of

But it is always good to be thorough :)