[Big-Daddy]

The following equilibria control the relative concentrations of molybdate and thiomolybdate ions in dilute aqueous solution. A solution initially containing 2.0×10^-7 M MoS₄^2- hydrolyzes in a closed system. The H₂S product accumulates until equilibrium is reached.

MoS₄^2- + H2O  MoOS₃^2- + H2S
MoOS₃^2- + H2O  MoO₂S₂^2- + H2S
MoO₂S₂^2- + H2O  MoO₃S^2- + H2S
MoO₃S^2- + H2O  MoO₄^2- + H2S

All ions and hydrogen sulphide are aqueous. Write the equilibrium equations that determine the system, noting the possibility that H₂S can ionize to HS^- and S^2- under certain pH conditions. (K values for all of these equilibria are available in tables)

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The equations that confuse me are the mass balances here. Molybdenum mass balance is an obvious summation over all species containing molybdenum, sulphur I don't get: I don't know if I should include the H₂S, HS^-, S^2- in the mass balance that starts from 8*10^-7, or not - because H₂S is produced as a side-product of the same reaction as produces the molybdate conjugates.