Qualitative Analysis

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Qualitative Analysis

 

Process Objectives

·    Experiment with qualitative tests in distinguishing between two unknown compounds.

 

Introduction

If an unknown sample is one of a limited number of possible compounds, a simple test often can be used to determine its composition. For example, a flame test can distinguish between KCI and NaNO3. A drop of potassium ferricyanide solution is useful in distinguishing FeCI2 from FeC13. There are hundreds of other simple “bench” tests that are useful in discriminating between a few possibilities. The alternative, sending a sample to a commercial analytical laboratory is expensive.

 

Qualitative analysis is very important to the forensic chemist, one who is interested in solving crimes. The analysis used in this experiment, however, has been largely replaced with instrumentation. For example, controlled substances, such as marijuana, now can be detected in blood almost one month after use. Instrumental analysis can detect as little as 1 x 10-11grams of 9-tetrahydro-cannabinol (one of the active ingredients in marijuana) in 1 mL of plasma. Unfortunately, this kind of instrumentation (a gas chromatograph connected to a mass spectrograph) is expensive, costing over half a million dollars. Often times the old bench chemistry is sufficient. Certainly, a drop of hydrochloric acid is enough for the police department’s forensic chemist to distinguish between a bag of cocaine and a bag of baking soda. Although, the price of this equipment is dropping and is becoming more and more available to the public, tests such as these will become commonplace at ones job or school.

 

In this experiment you are asked to identify the contents of a number of vials. The substance in each vial is one of the two compounds listed on the label. Decide which compound is present; you will make afew simple tests.

 

 Safety

Take the necessary precautions before beginning this exper­iment. Wear safety goggles, apron, and gloves. Read all safety precautions in your procedures and discuss them with your teacher. It is important to use good safety techniques while conduc­ting experiments..

   

Strategy for Experimenting

You now have a number of simple tests: flame tests, iron (II) and iron (III) tests; carbonate ion test, sulfite ion test and ammonium ion test.Use one of these simple chemical tests to identify each of the unknowns listed in the data table below.  If a solution is needed for a testsimply dissolve a small amount of the compound in water.

  

Data Table

 

Possible
Compounds

Test Used
Observations
Identity

1.   KNO3 or NaCI

 

 

 

2.  NH4CI or MgCO3

 

 

 

3.  FeCI3 or FeCl2

 

 

 

4.  K2S03 or KCl

 

 

 

5.  LiNO3 or’Na2CO3

 

 

 

6’ Sr(N03)2 or Na2SO3

 

 

 

7. (NH4)2C03 or ZnSO4

 

 

 

8.BaCI2 or NaNO3

 

 

 

9.Na2SO3 or Na2CO3

 

 

 

10.Fe(N03)3or Zn(N03)2

 

 

 

 

Questions

Strategy for Predicting

Consider all four ions in order to determine which test is appropriate. Review all of the tests. A useful test will give different results with each of the compounds. You should be able to predict the results for each compound before you begin the test.

 

 1.Explain how to generate ammonia gas in the laboratory. Write and balance the equation for the reaction.

 

 

 

 

2.Name a salt which will give a violet color to a flame and when HC1 is added will produce bubbles of gas which turn potassium permanganate brown.

 

 

 

 

General Conclusions

 

1.Which salt in the table will react with both HCI and NaOH? Show the equation for the reactions.

 

 

 

2.A few grains of an unknown white powder is brought into the crime laboratory ‘lb identify this substance should the bench chemical tests in this experiment be used? Explain.