We cannot multiply them as the amount of reactants dissociated in 2 steps may be different.
I will tell you what I have done.
Let the solubility of PbS be x moles/litre.
so Initially, Pb2+ and S2- formed will be x (they will change on further reaction)
x mole of Pb2+ gives y mole of Pb(OH)+ and H3O+
x mole of S2- gives z mole of HS- and OH-
z mole of HS- gives a mole of H2S
Since HS- and OH- is present in both equlibriums, HS- present is z-a and OH- is z+a
At equilibrium,
following are the concentrations LEFT.
PB2+ = x-y Pb(OH)+ and H3O+ = y
S2- = x-z HS-=z-a and OH- =z+a
HS-= z-a H2S=a and OH-=z+a
I hope you got it. Please tell me how to solve the equations.