1. The problem statement, all variables and given/known data http://i.minus.com/jfSl42bdxVH40.png [Broken] 2. Relevant equations pHO = -log[HO^-] 3. The attempt at a solution 1) Sulfide ion is a strong base so sulfide ion does not exist in water. Sulfide ion reacts with water upon contact to an extent of 100%. The reaction of sulfide ion with water forms hydroxide ion and hydrogen sulfide ion. So that teacher's work above is already wrong without any need for calculation as the teacher purports that there is a measurable quantity of sulfide ion in the system at equilibrium. [S^2-] should be ~0.