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VIRTUAL EXPERIMENT 2

Chemistry

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VIRTUAL EXPERIMENT 2. Mystery Bottles

 

Prelab Quiz: 

Watch ALL the associated videos and then complete the quiz. You won’t get access to the quiz until all content is viewed. No extensions granted on the quiz. 

 

Use the solubility rules in your lab assignment to make predictions. This quiz is meant to be done WITH your lab manual. In other words, it’s open book to encourage you to read all parts of the Mystery Bottles experiment. Hint: Fill in the mixing prediction table before starting the quiz. 

 

OBJECTIVE

 

The objective of this experiment is to use your knowledge about acid-base and precipitation reactions that occur in aqueous solutions to identify the ionic substances present in a set of solutions.  Flame tests will also be used to identify the content of the 7 mystery bottles.  

 

Course Objectives

• Recognize elemental symbols and names of the elements
• Name compounds from molecular pictures
• Analyze and solve problems in a consistent and organized manner
• Balance chemical reactions
• Understand some of the fundamental aspects of light

 

THEORY

 

The unknown solutions will be labeled A through G and you will be given the list of cations and anions that are present. All the solutions that you will be given are clear; hence you can already conclude that only cation/anion combinations that do not react with each other have been selected. Recall that when a cation and anion are mixed, a reaction occurs only if a precipitate is observed or a covalent compound (such as water or a gas) is formed.  In order to help you determine the unknowns, a set of solubility rules is required.  There are many versions of the rules but the following can be helpful since they are listed in hierarchical order.  For example, rule 1 takes precedence over rule 2 and so on. 

1. All Li⁺, Na⁺, K⁺, and NH₄⁺ salts are soluble.
2. NO₃⁻ (nitrate), C₂H₃O₂⁻ (acetate), ClO₃^- (chlorates) and ClO₄⁻ (perchlorate) salts are soluble.
3. Pb²⁺ and Hg₂²⁺ salts are insoluble.
4. SO₄²⁻ salts are soluble except for those of Ca²⁺, Sr²⁺ and Ba²⁺.
5. Ag⁺ salts are insoluble except for AgSO_4.  AgSO₄ is soluble only at low concentrations, as is the case for this experiment. 
6. Cl⁻, Br⁻ and I⁻ salts are soluble.
7. CO₃²⁻ (carbonate), S²⁻ (sulfide), O²⁻(oxide),  PO₄³⁻ (phosphate) and OH⁻ (hydroxide) salts are generally insoluble.

Thus, for example, using these rules we can predict that NaOH is soluble (Rule 1 takes precedence over Rule 7), that Pb(C₂H₃O₂)₂ is soluble (Rule 2 takes precedence over Rule 3) and that AgBr is insoluble (Rule 5 takes precedence over Rule 6).

As a simple illustration of the purpose of this experiment, suppose you are given three clear solutions X, Y and Z and are told that the solutions may contain the cations Na⁺, H₃O⁺ and Ag⁺ (each ion used once) and the anions CO₃²⁻, ClO₃⁻ and Br⁻. Since the solutions are clear, you may conclude that one solution does not contain AgBr since AgBr is insoluble in water and would form a precipitate (Rule 5 takes precedence over Rule 6).  Likewise, one solution does not contain Ag₂CO₃ since this compound is also insoluble (Rule 5 takes precedence over Rule 7). Thus, one solution must be AgClO₃. In a similar fashion, you know that none of the unknowns can contain both H₃O⁺ and CO₃^2-, because hydronium ion and carbonate ion react to form carbon dioxide gas and water according to the following equation 1:

 CO₃²⁻ (aq) + 2 H₃O⁺(aq) → H₂CO₃(aq) → CO₂(g) + H₂O(l) (1)

Thus, the others must be (H₃O)Br and Na₂CO₃. You could identify which is which by systematically mixing pairs of solutions and observing what happens. Suppose you mix X with Y and observe the formation of a pale yellow precipitate. Since Na⁺ and H₃O⁺ compounds are soluble, either X or Y must contain AgNO₃, and the reaction which occurs must be one of the following 2 or 3:

 2 Ag⁺(aq) + CO₃^2- (aq)  → Ag₂CO₃(s) (2)

 Ag⁺(aq) +  Br⁻(aq) →  AgBr(s) (3)

Suppose you now mix X with Z and also observe a yellow precipitate. Then, one of the above two reactions must have occurred; this identifies X as AgClO₃(aq). The identity of Y and Z is still unclear. If you mix Y and Z, you will observe bubbles due the reaction described in (1). We have no evidence at this point about Na⁺ and H₃O⁺. 

A flame test could be used to differentiate between these ions. In a flame test, when atoms of a metal are given extra energy (in this case placed into a flame), its electrons are promoted to higher energy levels.  When the electrons return to their original energy level, the excess energy is given off as photons.  The wavelengths given off are characteristic of the metal, just like the wavelengths produced by a gas discharge tube are characteristic of the specific element inside it.

Na gives a bright orange-yellow flame test, while H₃O⁺ gives no discernable color in a flame.  So, if Z burns orange-yellow, then it must be Na₂CO₃.  In addition, testing the unknown solutions with litmus paper can aid in our identification of the unknowns, as the presence of H₃O⁺ will cause blue litmus to turn red. So, if Y produces a red litmus test, it must be H₃OBr.  

 In your experiment, the unknown bottles are labeled I - VII.  These unknowns contain the following cations and anions: Ag⁺, Ba²⁺, Co²⁺, H₃O⁺, Li⁺, Na⁺, Sr²⁺ and Cl⁻, CO₃²⁻, OH⁻, NO₃⁻, SO₄²⁻. Each solution contains a different cation, but more than one solution can contain the same anion. Recall that a salt solution must contain both a cation and an anion.

 

 

PROCEDURE

 

Part A.  Solution Set- up 

1.  Obtain about 8 mL of each solution in separate medium test tubes. Label the test tubes carefully. If you pour out more solution than this, do not return any of it to the reagent bottle; just discard what is left over into the inorganic waste container at the end of your work. Place a clean glass Pasteur pipet in each test tube. 

 

Part B.  Litmus Tests

1. Test the acidity or basicity of each solution by dipping a clean glass stirring rod into the solution and placing a drop of the solution onto strips of neutral litmus paper. 

An acidic solution (H₃O⁺ present) will turn litmus paper red and a basic solution (OH⁻ or CO₃²⁻ present) will turn litmus paper blue.

 

Part C.  Mixing Solutions

1. Place one pipet full of one sample into each of 6 small test tubes.  
2. Test the reaction of each of the other 6 samples by adding one pipet full of each of the other solutions to one of the small test tubes.  

Observe any evidence of a reaction occurring, such as formation of a precipitate or evolution of a gas (bubbles). Record your observations.   

3. Test all seven unknowns filling in the observations table in detail.  Repeat any extraneous results.  

 

Part D.  Flame Tests  

1. Read Bunsen Burner TIPS in this manual and set up a Bunsen burner.  Do NOT leave the Bunsen burner unattended. 
2. Obtain a nichrome wire test loop, dip only the looped end carefully into the matching mystery bottle solution. (e.g.Wire I dipped in only Unknown I) The nichrome loops must be shared – be sure to return to beaker when done. 
3. Hold the looped end of the wire (not the handle) into the cool portion of the burner flame. Record the results, noting not only the color of the flame but also the intensity and duration. 

The colors of flames are: Li – carmine red; Ba – green; Na – orange-yellow; Sr – crimson red. The other ions give no characteristic flame test color.

4. Repeat steps #2-3 for each of the remaining 6 unknowns. Tests may have to be repeated. Get these observations checked by your IA/TA/lab coordinator before cleaning up. 

Every solution that has been stored in a glass vessel will give a positive Na test due to the leaching of sodium from the glass. Thus, a positive flame test due to a dissolved unknown Na salt should exhibit a persistent and very intense orange-yellow color. 

As a corollary, the colors due to other cations are visible briefly after the yellow of the leachedin Na disappears. Because you have other criteria besides flame tests for determining the ions present, no “known” solutions will be provided for comparison. 

Make some tentative hypotheses about which solution contains which cations based on the above results. Since each of the possible cations is present in one of the unknown solutions, you should observe a single occurrence of each of the flame colors described above.  Consider testing each solution more than once to confirm your hypothesis.  

 

Clean up: Glass Pipettes go in glass disposal containers only (white and blue box in the lab).  Pipette bulbs are returned to original location.   Dispose of all solutions in appropriate waste containers. Clean test tubes and return to bin.  

 

Data for Mystery Bottles:

There is no quantitative data for this experiment.  All of the qualitative data can be observed from the lab video (once released) and used for the fillable lab report.

             

LAB REPORT

 

A fillable lab report must be downloaded from MyLS and used to write this lab report. You cannot alter the order of the tables given in the document and marks will be deducted if you do not use the fillable document.  

You may add your own clearly labelled tables to the discussion only. 

No abstract is required for this lab report. There is some information and ideas below to assist with your lab report.

 

1. Remember, a precipitation reaction does not necessarily have to result in voluminous amounts of solid formed.  If the solution becomes cloudy, it contains a precipitate.
2. Each unknown contains an anion and a cation.  The cations are unique (ie not repeated) but the anions can be repeated, with all anions being used at least once.  
3. Draw conclusions from your flame results—remember that each cation is used once.
4. Draw conclusions based on your litmus tests.
5. Draw conclusions from your qualitative observations before mixing. 
6. The anion and cation in the same unknown can’t react with each other. (If they did, you would see precipitate in the unknown bottles.)  For example, if one of the unknowns contains Ba²⁺, you can create a mixing prediction table* to see that the only possible anions in that same unknown are Cl^- and NO₃^- since the remaining form a precipitate with Ba²⁺.  Fill in the chart that lists all seven cations with the possible anions that could be paired with it. 
7. Explain how you concluded each unknown identity with complete details, reactions, references to tables and evidence.   This makes up the majority of your discussion and lab report mark.    
8. Fill in the tables provided to organize your data. You will lose marks if you do not. 
9. NO sources of error or ways to improve the experiment, etc. are required.   

 

 

 

 

 

 

Fill in this mixing prediction table detailing what you predict will happen when these ions are mixed. This can be done using the solubility rules given in the theory. (NR = no reaction)

*Mixing Prediction table	Cl-	CO32-	OH-	NO3-	SO42-
Ag+
Ba2+	NR	BaCO3	Ba(OH)2	NR	BaSO4
H3O+
Li+
Na+		NR
Co2+
Sr2+

 

EXAMPLE of how to fill in this OPTIONAL table labelled #7 in your fillable document. If you need a starting point, this could be useful but is not required.

 

 

 

You have narrowed down that the unknown X should be Lithium which could have the nitrate, chlorite or sulfate ions paired with it.  You think the unknown Y is Calcium which could be paired with acetate or chlorate.  If X and Y react together, the negative ions of Y would be paired with Lithium and the negative ions of X would be paired with Calcium (double displacement (like a criss/cross) reaction). 

 

 IF the reaction between X and Y resulted in a precipitate, then Lithium must contain the sulfate ion in order for Calcium Sulfate to be formed as it’s the only precipitate possible out of all of the combinations. (Conversely, if a precipitate wasn’t formed, then sulfate could NOT be one of ions with Lithium). 

 

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