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Test for Cu2+, Fe3+, and Zn 6 M NaOH is added to a clear solution that may contain the three ions until the solution is basic. A dark precipitate forms. The precipitate totally dissolves in 6 M H2SO4. The addition of 6 M NH3 to this acidic solution until it is basic results in a clear solution containing a dark precipitate. The dark precipitate completely dissolves in 6 M H2SO4. Ions present Ions absent Ion undetermined

User Mighq
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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.

User Ddlab
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