Activity 10 | Qualitative Analysis

Purpose

To perform a qualitative analysis to identify the presence of three cations (Pb²⁺, Hg₂²⁺, and Ag⁺) using selective precipitation methods

Group I Cations

Pb²⁺ (lead), Hg₂²⁺ (mercury), and Ag⁺ (silver)

Dilute hydrochloric acid (HCl)

Used to precipitate Group I cations as chlorides (PbCl₂, Hg₂Cl₂, AgCl)

Identifying Pb²⁺

Identified by adding K₂CrO₄ (potassium chromate) to the filtrate. A yellow precipitate of PbCrO₄ confirms the presence of the ion

Identifying Ag⁺

Ammonia (NH₃) is added to the filtrate, causing Ag⁺ to form the soluble complex Ag(NH₃)₂⁺. Then, adding HNO₃ (nitric acid) forms a white precipitate of AgCl, confirming the presence of the ion

Identifying Hg₂²⁺

Adding ammonia (NH₃) to the filtrate containing Hg₂Cl₂ causes Hg₂²⁺ to form a black precipitate, confirming the presence of the ion

Precipitation of PbCl₂

The K_sp (solubility product constant) of PbCl₂ increases with temperature, causing it to dissolve in hot water, while AgCl and Hg₂Cl₂ do not dissolve

Solubility products (K_sp)

Indicates the solubility of a compound. A lower value means the compound is less soluble