Laboratory Experiment pubs.acs.org/jchemeduc
Identification of Unknown Chloride Salts Using a Combination of Qualitative Analysis and Titration with Silver Nitrate: A General Chemistry Laboratory Laina L. Maines and Martha D. Bruch* Department of Chemistry, SUNY Oswego, Oswego, New York 13126, United States S Supporting Information *
ABSTRACT: General chemistry students often have difficulty writing balanced equations and performing stoichiometry calculations for precipitation reactions, in part because of difficulty understanding the symbolic notation used to represent chemical reactions. We have developed a problem-based experiment to improve student learning of these concepts, and student surveys indicate this goal was achieved. Furthermore, this experiment requires students to integrate results from several different experiments and emphasizes critical-thinking and problem-solving skills. Students identify unknown chloride salts by (1) using a precipitation reaction to determine if the cation is a group 1 or group 2 metal, (2) performing a titration with silver nitrate to determine the formula mass of the salt, and (3) confirming the identity of the salt using a flame test. Students enjoyed the lab, and over 75% of the unknowns were correctly identified.
KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Precipitation/Solubility, Reactions, Stoichiometry, Titration/Volumetric Analysis
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formula mass with that calculated for each group 1 and 2 chloride salt, students identify the chemical formula for each unknown. Finally, to verify their identification, students predict the color that should be observed in a flame test, then perform the test to confirm that the observed color matches the prediction.
ymbolic notation is used extensively to represent chemical reactions, and general chemistry students must learn to form mental images of the atoms, molecules, and processes that this notation represents. Many students struggle with this process, which impedes their ability to write balanced chemical equations and perform stoichiometry calculations. Precipitation reactions are typically one of the first reactions encountered in general chemistry, and students often have difficulty using correct notation to write balanced chemical equations for these reactions. We propose a laboratory experiment, which emphasizes critical-thinking and problem-solving skills, to facilitate understanding of precipitation reactions and stoichiometry calculations. In the proposed experiment, students are given two unknown chloride salts, one with a group 1 cation and the other with a group 2 cation, and the main goal is to deduce the chemical formula of each unknown salt. This is done by combining the results from three separate experiments. First, the students mix aqueous solutions of the unknown salts with sodium phosphate solutions, observe whether a precipitate forms, and use solubility rules to decide if each unknown contains a group 1 or group 2 cation. Second, students dissolve a known mass of each unknown salt in water, titrate with silver nitrate of known molarity, then determine the formula mass of the salt from the titration data. By comparing the experimentally determined © 2012 American Chemical Society and Division of Chemical Education, Inc.
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EDUCATIONAL OBJECTIVES Other general chemistry labs incorporate precipitation reactions1 and flame tests2,3 for identification of salts, and precipitation titrations are often done in analytical chemistry labs. 4 However, the proposed lab represents a novel combination of quantitative and qualitative analysis that achieves a variety of goals simultaneously. It incorporates critical-thinking skills as students must resolve any discrepancies that arise between the expected and observed color in the flame test and involves problem-solving skills as students must determine the identity of an unknown salt. The experimental design also forces students to focus on interpretation of their own data to identify their unknowns. Because good results were obtained from five different chloride salts with a group 1 cation and two salts with a group 2 cation, Published: April 20, 2012 933
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oxidizing agents, and potassium chromate is a carcinogen, so care should be taken when handling these substances.
there are 10 possible combinations of unknowns where both types of cations are represented. Consequently, for a section of 20 students working in pairs, each group has a different combination of unknowns. The entire experiment can be completed in one lab period; students typically take two to three hours to complete all three parts. Hence, students cannot rely on the results of others to identify their unknowns, nor can they look up the answer in a textbook. Instead, students must integrate their own results from different experiments and make connections between different concepts discussed in class. Furthermore, using formula mass to identify the unknowns emphasizes the difference in stoichiometry between group 1 and group 2 cations reacting with silver nitrate and shows students that the formula mass of a salt is a quantity that can be determined experimentally. This experiment also helps students write balanced equations for precipitation reactions by providing concrete, visual examples that facilitate understanding of the symbolic notation used to represent chemical reactions. Finally, this experiment introduces students to titrations and allows students to learn and practice new lab skills; students have the opportunity to repeat the titrations as desired to obtain more accurate results.
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RESULTS AND DISCUSSION The silver nitrate solution was standardized by the instructor using four samples of sodium chloride, and the molarity was determined to be 0.1074 ± 0.0009 M, demonstrating that the titration gave accurate and reproducible results. Initially, five chloride salts with group 1 cations and four salts with group 2 cations were considered for use as unknowns, but two of the group 2 salts gave unreliable results when titrated and were not used. Magnesium chloride gave a formula mass that was too high compared to the theoretical value, suggesting that a significant quantity of water remained in the hydrated salt even after oven drying. Anhydrous magnesium chloride would likely give better results, but this was not tried. When a solution of barium chloride was titrated, the end point was reached prematurely, most likely due to reaction of barium chloride with the indicator to form barium chromate, a marginally soluble salt. This problem can be avoided if dichlorofluorescein is used as an indicator; results using potassium chromate versus dichlorofluorescein indicators are compared in the Supporting Information (Table S1). The remaining salts gave formula mass results that are within the estimated uncertainty of the theoretical value when titrated by the instructor (Table 1),
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EXPERIMENTAL OVERVIEW All chloride salts were obtained from Fisher Scientific, ovendried for 24 h at 130 °C, and stored in desiccators. A 0.1 M solution of silver nitrate was prepared by dissolving solid reagent-grade AgNO3 (Fisher Scientific) in water, deionized by filtering through two ion-exchange resin cartridges, and was standardized using dry reagent grade sodium chloride.5 Samples containing 0.5−1 g of the unknown salts were dispensed in 1.5 mL microcentrifuge tubes (Fisherbrand). Initially, students observed the precipitation reactions involved in the titrations. Next, students added 0.1 M sodium phosphate to aqueous solutions of each unknown and used the results to classify the cation as group 1 or 2. Students then dissolved a 0.1 g sample of salt into 25 mL of deionized water and added 0.5 mL of a 5% solution of potassium chromate as an indicator. This solution was titrated using the standardized silver nitrate solution until the end point was reached, characterized by the persistence of a red precipitate. Dichlorofluorescein could also be used as an indicator, which turns pink at the end point. The average experimental formula mass was determined from the titration data. The students calculated theoretical formula masses for each of the possible group 1 and 2 chloride salts and identified their unknowns by comparison of the experimental and theoretical formula masses. Finally, students conducted a flame test using a piece of nichrome wire and a Bunsen burner. A small loop was formed in the end of the wire; the loop was moistened in water and dipped into the tube containing the unknown salt. The wire was placed in the flame of the Bunsen burner, and color of the flame was compared to the expected color for the identified salt.
Table 1. Average Formula Mass for Chloride Salts Salt
Theoretical Formula Mass/ (g/mol)
Instructor Average Formula Mass/ (g/mol)
Student Average Formula Mass/ (g/mol)
LiCl NaCl KCl RbCl CsCl CaCl2 SrCl2
42.4 58.4 74.6 120.9 168.4 111.0 158.5
42 ± 1 58 ± 2 74 ± 1 120 ± 4 170 ± 3 117 ± 2 154 ± 6
43 ± 2 57 ± 19 76 ± 8 121 ± 5 156 ± 16 116 ± 18 160 ± 10
except for calcium chloride. This is probably due to the tendency of this salt to absorb water, which would lead to a measured formula mass higher than expected. The experiment was performed with three sections of 20 students each. The formula mass obtained by each student was tabulated from the lab reports submitted after the experiment was completed, and the average results for each salt are summarized in Table 1. The average formula mass obtained by students is nearly identical to that observed by the instructor, except for cesium chloride. The low average observed by students is likely due to the small volume of titrant needed to reach the end point (approximately 6 mL). Inexperienced students tend to go past the end point, leading to a low value for the molar mass. Although the standard deviation on the student results is higher than that obtained by the instructor for the same salts, for most salts the relative standard deviation (RSD) on student data is 10% or less. The notable exceptions are calcium chloride, with a RSD of 16%, and sodium chloride, with a 33% RSD. The large standard deviation for sodium chloride has no obvious explanation as this is the salt used to standardize the silver nitrate, resulting in a RSD of less than 1%. Nevertheless, for most students, the measured formula mass was close enough to the true value to correctly identify the salt. Misidentified salts become obvious to students when they
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HAZARDS Silver nitrate, potassium chromate, barium chloride, and the precipitates from their reactions are toxic and must be disposed of as hazardous waste. All reagents and chloride salts are irritants that can cause skin and eye burns, so gloves, safety glasses, and lab coats should be used to avoid contact with skin, eyes, and clothing. Silver nitrate and potassium chromate are 934
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perform the flame test, providing students an opportunity to critically analyze all the data and resolve discrepancies between the expected and observed colors by repeating experiments as necessary. By utilizing all the data, over 75% of the unknowns were correctly identified by students. The effectiveness at achieving the objectives for the experiment was assessed by a student survey, summarized in the Supporting Information. When asked what they had learned from the lab, 40% of students felt the lab helped them to write chemical formulas, over 60% said it helped them understand precipitation reactions, and over 80% said it helped them understand titrations and flame tests (Table S2). When asked to compare the experiment to other labs they had performed, 55% said they learned the same amount as with other labs and 36% said they learned more in this lab. The difficulty of the lab was rated the same as other labs by 50% of the students and easier by 40%. An effective problem-based experiment requires students to demonstrate critical-thinking skills, apply concepts to new situations, and develop and use problem-solving strategies, and at least 75% of students surveyed felt each of these skills was important for this lab (Table S3). Finally, students enjoyed this lab: 73% said it was more fun than other labs, 82% said it was a nice change of pace, and 89% would recommend it to a friend studying chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
Indicator comparison; student survey results; notes for the instructor; directions for the students. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Abraham, M. R.; Pavelich, M. J. Inquiries into Chemistry, 3rd ed.; Waveland Press: Long Grove, NY, 1999; pp 214−232. (2) Bare, W. D.; Bradley, T.; Pullman, E. J. Chem. Educ. 1999, 75, 459. (3) Sanger, M. J.; Phelps, A. J. J. Chem. Educ. 2004, 81, 969−970. (4) Butler, E. A.; Swift, E. H. J. Chem. Educ. 1972, 49, 425−427. (5) Skoog, D. A.; West, D. M.; Holler., F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 8th ed.; Brooks/Cole: Belmont, CA, 2004; pp 337−363.
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