Experiment 75

Objective

To identify the presence of zinc in a sample by observing the reactions of zinc ions with sodium hydroxide and hydrogen sulfide.

Introduction

Zinc sulfate reacts with sodium hydroxide (NaOH) to form a white precipitate of zinc hydroxide (Zn(OH)₂). When excess NaOH is added, the precipitate dissolves, forming sodium zincate (Na₂ZnO₂). Passing hydrogen sulfide (H₂S) gas through the sodium zincate solution results in the formation of a white precipitate of zinc sulfide (ZnS).

Reactions:

1.    Formation of zinc hydroxide:

ZnSO4 + 2NaOH → Na2SO4 + Zn (OH)2 ↓

2.    Formation of sodium zincate:

Zn(OH)2 + 2NaOH → Na2ZnO2 + 2H2O

Formation of zinc sulfide:

Na2ZnO2 + 2H2S → ZnS ↓ + Na2S + 2H2O

Materials

Chemicals

·       Supplied sample containing zinc salt

·       Sodium hydroxide (NaOH)

·       Hydrogen sulfide (H₂S)

Apparatus

·       Test tube

·       Test tube holder

·       Bunsen burner

Reagents

·       Dissolve 3.657 g of the sample in 250 ml of distilled water.

Procedure

1.    Place 5–6 ml of the sample solution in a test tube.

2.    Add 3–4 ml of sodium hydroxide solution to the sample.

3.    Observe the formation of a white precipitate.

4.    Add an additional 3–5 ml of NaOH to dissolve the precipitate, forming a clear solution.

5.    Pass hydrogen sulfide gas through the solution and observe the formation of a white precipitate.

Observation

A white precipitate of Zn(OH)₂ forms upon the addition of NaOH. This precipitate dissolves with excess NaOH, and when H₂S gas is passed through the solution, a white precipitate of ZnS is formed.

Questions

1.    What is sodium zincate?

2.    What happens when NaOH is added to the zinc solution?

3.    What happens when excess NaOH is added to the zinc solution?

4.    How can zinc be identified in a salt?