In the Classroom
An Ionic Compound Logic Puzzle Craig P. McClure Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294;
[email protected] A number of puzzles have been published in this Journal in a variety of formats including sudoku puzzles, logic puzzles, and others in an effort to introduce or reinforce concepts discussed in chemistry courses (1–4). These puzzles add a variety of activities to a course. Additionally they may be used to emphasize that chemistry is not focused on memorizing facts, but instead on applying knowledge and logic to solve problems. The logic puzzle presented here requires students to apply the concepts of ionic compound empirical formulas, ions present in aqueous solutions, the formation of precipitates, and polyatomic ions. Several terms are also reinforced by their application in solving the puzzle. This puzzle is appropriate for general chemistry students of all levels and may aid in integrating these concepts in the students’ minds. This activity is suitable for individual or group work. Puzzle Instructions Five students (Alexander, Benjamin, Cierra, Drew, and Erin) are given unknown aqueous solutions, each containing a different ionic compound. The ionic compounds contain ammonium, calcium, nickel, potassium, or zinc as the cation and bromide, fluoride, hydroxide, nitrate, or sulfate as the anion. No single ion is repeated. Using the clues below, determine the ionic compound contained in each student’s solution and its molar mass.
1. Benjamin’s compound has a molar mass greater than that of the one containing the hydroxide ion, but smaller than the one containing the ammonium ion. The compound with the largest molar mass has a cation:anion ratio of 1:2.
2. Cierra’s compound formed a precipitate when aqueous sodium carbonate was added. The compound with the middle value for molar mass did not contain a divalent cation. The compound with the second smallest molar mass formed a precipitate with aqueous barium iodide.
3. The molar mass of Drew’s compound is between 50 and 100 grams per mole greater than the compound containing nickel. One compound had a cation:anion ratio of 2:1.
4. Alexander’s compound contains all polyatomic ions. Erin has the compound with the smallest molar mass.
5. Benjamin’s compound contains a transition metal. The compound containing zinc had the largest molar mass. The compound with the second largest molar mass formed a precipitate when aqueous lithium sulfate was added.
6. A precipitate was formed when a solution of iron(II) chloride was added to the solution containing the compound with the smallest molar mass. The largest molar mass of a compound is over 200 grams per mole.
Table 1. Solubility Chart for Combinations of Ions
Table 2. Logic Puzzle Solving Grid
Nickel(II)
Potassium
Sodium
Zinc
aq
aq
aq
aq
aq
aq
Bromide
aq
aq
aq
aq
s
aq
aq
aq
aq
aq
s
s
s
s
aq
s
aq
aq
s
Carbonate aq Chloride
aq
aq
aq
aq
s
aq
aq
aq
aq
aq
Fluoride
aq
s
s
s
s
aq
aq
aq
aq
aq
Hydroxide
aq
aq
s
s
s
aq
s
aq
aq
s
Iodide
aq
aq
aq
aq
s
aq
aq
aq
aq
aq
Nitrate
aq
aq
aq
aq
aq
aq
aq
aq
aq
aq
Phosphate
aq
s
s
s
s
s
s
aq
aq
s
Sulfate
aq
s
s
aq
s
aq
aq
aq
aq
aq
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sulfate
Lithium
aq
Anion
(high) bromide fluoride hydroxide nitrate
Lead(II)
aq
Molar Mass
ammonium calcium nickel(II) potassium zinc(II) (low)
Iron(II)
aq
Alexander Student
Calcium
aq
Benjamin Cierra Drew Erin bromide
Anion
Barium
Acetate
fluoride hydroxide nitrate sulfate
Molar Mass
Ions
Ammonium
Cation
(low)
(high)
Journal of Chemical Education • Vol. 86 No. 10 October 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Classroom
A solubility chart (Table 1) (5) is given to the students along with the clues. Compounds with a solubility in cold water greater than 1.0 g/100 mL are designated as soluble (aq), while insoluble compounds remain solid (s). A solving grid is also available (Table 2) that may be given to the students to aid in solving the puzzle. In the solving grid, an “X” is normally placed on a combination that is not possible, given the clues presented, while an “O” is used to signify a confirmed relationship. For example, since all of the unknown solutions are aqueous, an “X” may be placed at the intersection of the nickel column and hydroxide row, since nickel hydroxide is insoluble and therefore may be ruled out as an unknown. Once the anion paired with nickel is confirmed from the puzzle clues, an “O” would be placed in the square where nickel and its anion intersect. After the identity of each unknown is obtained, each student’s unknown compound and its molar mass can be placed in the Solution Reporting Table (Table 3), for easy comparison to the puzzle solution (Table 4). Summary This logic puzzle is designed to promote logical thinking skills in the application of chemical concepts in an enjoyable and challenging format. This activity also serves as a review of many topics related to ionic compounds, and synthesis of these ideas is necessary for the successful completion of the activity. A student-ready version of this activity is available online for ease in integration of the activity into the classroom. Literature Cited 1. Boyd, S. L. J. Chem. Educ. 2007, 84, 619–621. 2. Castro-Acuña, C. M.; Dominguez-Danache, R. E.; Kelter, P. B.; Grundman, J. J. Chem. Educ. 1999, 76, 496–498. 3. Peris, M. J. Chem. Educ. 2007, 84, 609. 4. Welsh, M. J. J. Chem. Educ. 2007, 84, 610–611. 5. CRC Handbook of Chemistry and Physics, 74th ed.; Lide, D. R., Ed.; CRC Press, Inc.: Boca Raton, FL, 1993; pp 4–36, 4–114.
Table 3. Solution Reporting Table Student
Unknown Compound
Molar Mass/(g/mol)
Alexander Benjamin Cierra Drew Erin
Table 4. Puzzle Solution Student
Unknown Compound
Molar Mass/(g/mol)
Alexander
(NH4)2SO4
132.15
Benjamin
NiF2
Cierra
ZnBr2
225.19
Drew
Ca(NO3)2
164.10
Erin
KOH
96.69
56.11
Supporting JCE Online Material http://www.jce.divched.org/Journal/Issues/2009/Oct/abs1210.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Student-ready version of the puzzle
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 10 October 2009 • Journal of Chemical Education
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