Final answer:
Based on the chemical tests and observed reactions with NaOH and NH3, Zn2+ is confirmed to be present, while Fe3+ and Cu2+ cannot be conclusively identified from the given data.
Step-by-step explanation:
The student is conducting qualitative analysis to test for Cu2+, Fe3+, and Zn2+ ions using sodium hydroxide (NaOH) and ammonia (NH3). Here's a summary of the observations and conclusions:
- After adding 6 M NaOH to a clear solution, a dark precipitate forms. This indicates the presence of ions that form base-insoluble hydroxides.
- The precipitate dissolves completely upon adding 6 M sulfuric acid (H2SO4), suggesting it is not a sulfide precipitate which wouldn't dissolve in acid.
- The addition of 6 M NH3 to the acidic solution forms a clear solution with a dark precipitate. Fe2+ typically forms a precipitate under these conditions which would not dissolve in excess H2SO4. Therefore, the ion could be Fe3+ that was reduced to Fe2+ and then reoxidized; Cu2+ or Zn2+ also form complex ions with NH3 which do not precipitate.
- Finally, the precipitate once again dissolves in 6 M H2SO4, affirming that it can form complex ions with NH3 and is soluble in strong acids.
The evidence suggests Zn2+ is present due to its characteristic reaction with NaOH forming a white precipitate and its solubility in excess NaOH or NH3. However, Fe3+ and Cu2+ cannot be conclusively identified or excluded based on the given observations.
In conclusion, Zn2+ is present, but the presence of Fe3+ and Cu2+ is undetermined from the given data.