7 Exp. 7: Testing the Solubility Rules Through Quantitative Analysis

Pre-Lab: Use BB to quiz on the following before coming to lab.

1. Determine the amount of solid NaBr required to make 10. mL of a 0.20 M solution.

2. Find the spectator ions in the following precipitation reaction:

2NaI(aq) + Pb(NO3)2(aq) à 2NaNO3(aq) + PbI2(s)

3. Find an anion that is soluble with silver cations.

4. Why must you use gloves and wash your hands when working with heavy metals such as Pb and Cd?

Introduction:

In the Density Determination and Percent Composition from Gravimetric Analysis labs, you performed quantitative analysis by finding the amount of an analyte contained in a sample. This lab uses qualitative analysis to identify a substance through its solubility but does not quantify the substance. Qualitative analysis is used in fields such as forensics, arson and explosive investigation, and drug identification, among others. Physical and chemical properties, such as color, smell, fluorescence, reactivity, melting point, solubility, etc. can be used in identifying a chemical species. Cations and anions will be identified qualitatively in this lab by aqueous solubility.

Svante Arrhenius, 1903 Nobel prize winner and great grandfather of Greta Thunburg, is credited as the first person to predict that carbon emissions in the form of CO2(g) were large enough to change earth’s climate. He won the Nobel prize in 1903, however, for his work describing the dissolution of ionic compounds and the stability of the resulting ions through ion-dipole interactions. Svante’s description of ions separating in water is elegantly simple and widely accepted now. At the time, the theory was poorly received by his professors. He in fact narrowly passed his graduate thesis defense on the topic.

Not all ionic compounds are soluble in water, giving rise to solubility tables like the one you will use in this lab. Some ionic solubilities are clear cut. AgNO3, for example is very soluble while AgCl is not. Some solubilities are more nuanced. Ag2SO4, for instance, is listed as an exception to the rule that sulfates are generally water soluble. In fact, five grams of silver sulfate will dissolve in one liter of water. Solubility is most accurately described quantitatively. Tabulating solubility qualitatively provides clear and easy to use rules that allow for the qualitative identification of unknown cations and anions. The designations of soluble or insoluble, however, are arbitrary, giving rise to discrepancies even among test books. The second semester of this course uses k values or ratios of dissociated ions to the solid compound to quantitatively describe solubility but is beyond the scope of this lab.

*Notable Complication: Ag2SO4 is moderately soluble. See introduction.

Precipitation Reactions

By using the solubility rules the products of a double replacement reaction can be predicted. The molecular reaction lists all species as (aq) aqueous or (s) solid. No ions are shown in the molecular reaction. The total ionic equation lists all aqueous species as ions, and the net ionic equation excludes spectator ions. Let’s look at an example of each using sodium iodide and lead (II) nitrate. Please note that precipitation double replacement reactions will start with two aqueous compounds.

Molecular:                    2NaI(aq) + Pb(NO3)2(aq)à 2NaNO3(aq) + PbI2(s)

Total Ionic:                       2Na+ + 2I- + Pb2+ + 2NO3- à 2Na+ + 2NO3- + PbI2(s)

Net Ionic:                           2I- + Pb2+ à PbI2(s)

Objective:

The objective of this lab is to identify an unknown set of cations and an unknown combination of one to two anions using precipitation reactions.

Safety:

Barium and Lead ions are toxic. Use gloves and hand washing to minimize exposure to these chemicals. Wash your hands before leaving the lab, eating, or drinking. 0.1 M NaOH is a strong base. Use caution and wear splash goggles.

Waste/Housekeeping:

All solutions will be disposed of in the same waste in hood #1.

Avoid contaminating solutions. Do not return any solution to its container. Use the beral pipettes located in the solution holsters and return them without touching the counter.

Procedure:

Preparation of a 0.2 M NaBr solution

1. Calculate and weigh enough NaBr(s) to make 10 mL of a 0.2 M NaBr solution. Use your smallest Erlenmeyer flask and a graduated cylinder to approximate the molarity to one significant figure.
2. The other three anions, 0.1 M Na2SO4, 0.1 M Na3PO4 and 0.1 M NaOH, can be distributed dropwise with a pipette.

*Note that the anion is of interest. Sodium is the counter ion and will always dissociate.

Cation Analysis

*The letters used to label solutions are repeated in this lab. Be sure to use the solutions marked Cation for this step.

3. The cations K+, Cu2+, Ag+, Ba2+, Fe3+, Mg2+, and Pb2+ are randomly designated A – G. Each unknown cation is paired with nitrate ion, NO3-, as a counter ion. Arrange your test tubes in such a way as to test each cation, A – G with each of the four known anions from steps 1 and 2. Add or approximate 10 -12 drops of each cation and anion to test all possible combinations. For example, you might arrange a column of four ‘A’ cations, four ‘B’ cations and so on, then add Br- anion across the entire row. Create your own system, but label or keep notes to track your observations. Be sure to use one beral pipette for each solution and add the solutions without touching the sides of the test tube. It is better to waste some solution than to risk contamination.
4. Record your observations such as no change, precipitation, color, etc.
5. Identify your unknown cations, A – G. Use all your observations, including the color of the unreacted reagent.
6. During or after lab, write molecular, total ionic, and net ionic equations for each precipitation reaction that you observe from your cation analysis. Combinations with no observable change will not yield a net ionic reaction and may be ignored.

The following sample data table may help you track your observations. Be sure to modify it based on your procedure.

Identify cations A – G.

Anion Analysis:

*The letters used to label solutions are repeated in this lab. Be sure to use the solutions marked Anion for this step.

Now that you have found the identities of cations A – G, you can use these identities along with the solubility rules to find the identity of one unknown anion or anion combination. You will be assigned one anion unknown randomly lettered A – D from the following possibilities: Br-, Br-/SO42-, Br-/PO42-, Br-/OH-.

Combine 10-12 drops of your unknown anion with your known cations A – G to find the identity of your unknown anion(s).

The following sample data table may help. You can now label your cations based on their identities.

Anion letter: ___________

Identify your anion(s): _______________

Post Lab:

1. Consider a solution that contains a mixture of Ba2+ and Fe3+.
   1. What anion would you add to precipitate only Fe3+?
   2. What anion would you add to only precipitate Ba2+?
2. In what way is the title of a table, ‘Solubility Rules’ a misnomer? How would you better define solubility.
3. Find ‘rules’ from two sources and list at least one contradiction among the rules. You may use this lab or your text as one of the sources.
4. Find and describe an outside application of precipitating one cation or anion while leaving another or others in solution, i.e., selective precipitation.

Your Report:

1. Complete the pre-lab and take the pre-lab quiz in Blackboard prior to attending lab.
2. Use a lab notebook to take notes and make observations.
3. Create an informal lab document to turn in at the beginning of the next lab. Your lab report should have your name, letter, and identity of unknown anion, clearly labeled collected data in tables and or graphs where applicable, and molecular, total ionic, and net ionic equations of all precipitations. Think about the main point of the lab, or the results that you worked for and be sure to include it or them.
4. Numbered responses to any post lab questions. You don’t need to re-write the questions.
5. Submit your document before your next lab appointment under the assignment tab on your laboratory Blackboard shell.

References:

Adapted from Dieckmann, G., Sibert, J. In An Atoms First Approach to the General Chemistry Laboratory; McGraw Hill: New York, NY, 2014; pp 81–96.

The Nobel Prize in Chemistry 1903. https://www.nobelprize.org/prizes/chemistry/1903/arrhenius/biographical/ (accessed Jul 1, 2020).