Chemistry ????

Lab #15: Separation and Qualitative Analysis of Cations

Purpose

In this experiment, you will use analyze the characteristic reactions that a variety of cations undergo. You will then devise a qualitative analysis scheme to analyze a solution that contains a variety of these cations in one solution. You will then use this analysis to determine the identity of an unknown cation solution.

Introduction

When a chemist conducts a qualitative analysis, the primary goal is to identify the substances that are present in a mixture. It is important to separate substances from each other in a mixture, so that a confirming test can be as definitive as possible. After a thorough qualitative analysis investigation, it would be nice to say, for example, that potassium is present instead of saying merely that an alkali metal is present.

Conducting a qualitative analysis, or “qual scheme” as it is sometimes nicknamed, is a methodical process that requires great care. A qual scheme can be used to identify a wide variety of substances; in this experiment, you will focus on ions. In this unit you have been learning about solubility equilibria and complex-ion equilibria, both of which play crucial roles in the separations and identifications that are part of a qual scheme. You will see some very interesting things during this experiment, things that will help you understand more about the nature of cations and anions.

Reaction with HCl

A common step in qualitative analysis schemes is to react with chlorides. Usually only a few cations, Pb2+, Hg22+, and Ag+, will react to form precipitates with chloride ions. It is useful to use reagants that only react with a few cations to more easily narrow down the identity of some cations.

In this lab you will be testing Pb2+ and Ag+. The way you can determine the difference between the two ions is by adding NH3 to the precipitate. AgCl will dissolve in a basic solution but PbCl2 will not.

Reaction with NH3

Ammonia is a unique reagent because it will show a variety of products. Sometimes a precipitate occurs with a characteristic color, but in other cases a complex ion is formed with a very vibrant color change. Therefore it is useful to record all changes that you observe.

Reaction with KOH and Na2CrO4

Certain cations will react with KOH or Na2CrO4 to form an insoluble precipitate. The precipitates will have a characteristic color which helps to identify which cation is reacting. You will study the reaction for each ion separately, and then carefully record all observations about the reaction.

Development of a qualitative analysis scheme

The final objective of this lab is to take the knowledge you have learned and use it to separate and identify a mixture of cations. To try to separate ions, you will need to be thoughtful about the order you do the tests. Also, it is often difficult to completely separate an ion once it has been identified. The procedure will help with this as it explains how to use a centrifuge for separating liquids and solids. The liquid that is left after a precipitate is separated is referred to as the supernatant. Also, there may be situations where you cannot do a complete separation, but at least narrow down the options for the ions that may be present.

Procedure

1. This lab is an online simulation. Go to the website: https://static-archives.git-pages.mst.edu/userweb-gbert/qual/qual.html

2. Instruction for how to operate the simulation will be on the right side of the screen. Please read carefully so you know how to operate the equipment in the simulation.

Part I: Analyze characteristic reactions for Ag+, Hg22+, Pb2+

3. Select Ag+ from the Sample menu. Put 2 drops in the first test tube. Select Hg22+ from the Sample menu. Put 2 drops in the second test tube. Select Pb2+ from the Sample menu. Put 2 drops in the third test tube.

4. Select 3 M hydrochloric acid from the Reagent menu. Add 2 drops of 3 M HCl solution to each test tube and record your observations.

5. Centrifuge the test tubes by clicking on the centrifuge button. Decant and discard the supernatant liquid from each tube (click decant and then click on the waste container to discard.)

6. Wash the precipitates by adding 2 drops of distilled water to each tube. Heat the test tubes in a boiling water bath (click on heat). Record your observations.

7. Add two drops of ammonia (NH3) solution to each tube, and test the acidity of each solution by dipping a clean stirring rod into the liquid and touching it to litmus paper.  If the solutions are not basic, add more ammonia until they turn red litmus blue.  Record any changes in the precipitate.

8. Discard all of the solutions in the waste container.

9. Select Ag+ from the Sample menu. Put 2 drops in the first test tube. Select Hg22+ from the Sample menu. Put 2 drops in the second test tube. Select Pb2+ from the Sample menu. Put 2 drops in the third test tube.

10. Select 3 M KOH from the Reagent menu. Add 2 drops of KOH solution to each test tube. Centrifuge and record your observations.

11. Discard all of the solutions in the waste container.

12. Select Ag+ from the Sample menu. Put 2 drops in the first test tube. Select Hg22+ from the Sample menu. Put 2 drops in the second test tube. Select Pb2+ from the Sample menu. Put 2 drops in the third test tube.

13. Select 1 M Na2CrO4 from the Reagent menu. Add 2 drops of Na2CrO4 solution to each test tube. Centrifuge and record your observations.

14. Discard all of the solutions in the waste container.

Part II: Analyze an unknown solution

Note: This part of the procedure is purposely open-ended and requires you to use results and critical thinking to determine what the steps will be. Be sure to record everything that you do.

15. Select Trial Unknown from the Sample menu. Add 2 drops to a test tube. The unknown is randomized for every student so don’t expect your results to be the same as another student’s.

16. Choose a reagent to help you determine the identity of the known. Record your choice in the results section.

17. Add the reagent and record your observations.

18. Repeat Steps #15 - #18 with other reagents until you have confirmed the identity of the unknown. Take a picture of your test tubes and include with your report.

Name __________________

Pre-laboratory Assignment

1. Define qualitatitive analysis. How will it be used in this experiment?

2. The following terms are used to describe substances in this lab. Define the terms using the lab introduction and your textbook:

a. Precipitate

b. Supernatant

c. Centrifuge

3. What are the formulas of:

a. The cations that will be tested in this experiment?

b. The reagents that will be used to separate the cations in this experiment?

Results

Test Tube	Cation	HCl	HCl + heat	HCl + NH3	KOH	Na2CrO4
1	Ag+
2	Hg22+
3	Pb2+

Test Performed on Unknown	Results

Questions

1. What is the cation that is present in your unknown solution?

2. Describe, in detail, how you used the results of your testing to identify the contents of your unknown solution.

3. Show precipitation reactions for the following reactions you performed in this simulation. Remember to indicate which product is the solid. (See Chapter 7 if you need a reminder on how to write precipitation reactions)

a. Pick one of the cations and show the reaction with HCl

b. Pick one of the cations and show the reaction with KOH

c. Pick one of the cations and show the reaction with Na2CrO4

Lab #15

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