Chapter 17, Problem 17.138SP

Interpretation Introduction

Interpretation:

The test to identify three white solids $\text{NaCl}$, $\text{KCl}$ and ${\text{MgCl}}_2$ should be determined.

Concept introduction:

Metal cations can be separated into two groups by the precipitation of metal sulfide. The cations which form very insoluble sulfides can be separated from cations which form soluble sulfides. The separation takes place in an acidic solution and uses solubility equilibrium.

The separation depends on the ${\text{H}}_{\text{3}}{\text{O}}^{+}$ concentration so that the reaction quotient ${\text{Q}}_{\text{c}}$ exceeds $K_{\text{spa}}$ for insoluble sulfides but not for soluble sulfide. As a result, insoluble sulfides precipitate under acidic condition but soluble sulfides remain in solution.

Qualitative analysis identifies ions present in an unknown solution. The ions are identified by chemical tests. In the scheme of analysis, some 20 metal cations are separated into five groups by selective precipitation.

The flowchart that shows the separation of metal cations in qualitative analysis is as follows: