A solution which is 10-2 M each in Mn2+ ; Fe+2, Zn2+ ; Hg2+ is treated with 10−20 sulphide ion. If Ksp MnS ; FeS ; ZnS ; HgS are 10−13, 10−18, 10−24,10−53    respectively. Which one will precipitate first?

When ionic product predominates solubility product; then precipitation takes place.Ionic product of HgS=Hg2+S2−=10−210−20=10−22 < Ksp (10-53) The metal sulphide having the lowest solubility will precipitate first provided the ionic product is higher than the Ksp​. Here, all salts are of the same valence type. So, the sulphide having the lowest Ksp​ value will precipitate first provided