Neco 2025 Chemistry Question 19

A white crystalline solid is dissolved in distilled water to give a clear solution. The following observations are made:\n\n1. A copper‑green flame is observed when a small portion of the solution is heated with a copper wire.\n2. Adding a few drops of 0.10 M NaOH produces a pale blue precipitate which dissolves on addition of excess NaOH, giving a deep blue solution.\n3. Adding dilute HCl followed by \(\mathrm{H_2S}\) gas yields a black precipitate that does not dissolve on heating with dilute acid.\n4. Adding ammonium chloride solution and then excess NH\(_4\)OH gives a deep blue solution.\n\nA 25.0 mL aliquot of the solution is treated with excess 0.10 M NaOH, the precipitate is filtered, washed and dried to give 0.215 g of copper(II) hydroxide. Identify the cation present and determine its concentration (mol L\(^{-1}\)) in the original solution.