When H2S dissociates in water, it forms 2H+ and S2-. This means the expression of Ka [H+]^2[S2-] / [H2S]. H+ concentration can be derived from the pH thru 10^-pH. This is equal to 1.7783 x10^-4 M. Ka now becomes [1.7783 x10^-4 M]^2*[1.7783 x10^-4 M/2]/ [1.0-1.7783 x10^-4 M] equal to 2.8122 x10^-12.