Question

What is the weak base ionization constant (`K_b`) for `HS^-` equal to?

Answer 1

Well `pK_b=10^(14-pK_a)`............; and thus `K_b=1.25xx10^-7`............

Explanation:

Hydrogen sulfide dissociates according to the reaction......

`H_2S(aq) + H_2O rightleftharpoonsHS^(-) + H_3O^+`

Now `K_a(H_2S)=10^(-6.9)=1.25xx10^-7`, and of course `pK_a=6.9`.

In aqueous solution, we know that `pK_a+pK_b=14`

And thus `pK_b=14-pK_a=14-6.9=7.1`

`K_b=10^(-7.1)`...........`=??`

Note that the question SHOULD have quoted at least `pK_(a1)` for `H_2S`. `K_a` and `K_b` are established by measurement; you could not possibly be expected to remember them.