Five students (Alexander, Benjamin, Cierra, Drew, and Erin) are given unknown aq
ID: 528084 • Letter: F
Question
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. 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 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. 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 Alexander's compound contains all polyatomic ions. Erin has the compound with the smallest molar mass. 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. 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 m mass of a compound is over 200 grams per moleExplanation / Answer
Note the cations and the anions and the atomic masses of the elements.
Cation
Atomic Mass
Anion
Atomic Mass
Ammonium, NH4+
17.00
Bromide, Br-
79.90
Calcium, Ca2+
40.08
Fluoride, F-
19.00
Nickel, Ni2+
58.69
Hydroxide, OH-
17.00
Potassium, K+
39.10
Nitrate, NO3-
62.00
Zinc, Zn2+
65.38
Sulfate, SO42-
96.00
1) The compound with the highest molar mass will be formed by the elements with the highest atomic masses. The compound with the highest molar mass is zinc bromide, ZnBr2 having a molar mass of (65.38 + 2*79.90) = 225.18. Also, the compound with the highest molar mass, ZnBr2 has cation:anion = Zn:Br = 1:2.
2) The compound with the smallest molar mass (from the table above) is ammonium hydroxide, NH4OH, having a molar mass of (17 + 17) = 34. However, we know that ammonium and hydroxide ions are not contained in the same compound (see clue 1). Therefore, NH4OH isn’t a possible compound.
3) The compound with the smallest molar mass forms a precipitate with a solution of ferric chloride, FeCl3. Therefore, the compound with the smallest molar mass must have hydroxide, OH- and the precipitate formed is that of iron (III) hydroxide. The compound with the smallest molar mass and one which must contain OH- is potassium hydroxide, KOH and the molar mass is (39.10 + 17) = 56.10. This compound belonged to Erin. The precipitation reaction is
FeCl3 (aq) + 3 KOH (aq) ------> Fe(OH)3 (aq) + 3 KCl (aq)
3) Alexander’s compound had all polyatomic ions. The only polyatomic cation is NH4+ while the available polyatomic anions are NO3- and SO42-. One of the compounds had a cation:anion ratio of 2:1, i.e, 2 atoms of the cation combine with 1 atom of the anion. The only possibility (since K+ is already assigned to Erin) is NH4+ as the cation and the compound is ammonium sulfate, (NH4)2SO4. The molar mass of(NH4)2SO4 is (2*17 + 96) = 130. (NH4)2SO4 was assigned to Alexander.
4) The cations and anions left are Ca2+, Ni2+, F-and NO3-. The possible compounds are CaF2, Ca(NO3)2, NiF2 (highly unlikely to form) and Ni(NO3)2.
5) Benjamin’s compound contained a transition element. Since nickel is the only “true” transition element present, therefore, Benjamin must have got nickel nitrate, Ni(NO3)2. The molar mass of the compound is (58.69 + 2*62) = 182.69. This is also the compound with the second largest molar mass.
6) The only option left is CaF2, having a molar mass of (40.08 + 2*19.00) = 78.08. This compound was assigned to Cierra. This is also the only compound which gives a precipitate with aqueous sodium carbonate, Na2CO3. The precipitate is due to calcium carbonate, CaCO3.
Cation
Atomic Mass
Anion
Atomic Mass
Ammonium, NH4+
17.00
Bromide, Br-
79.90
Calcium, Ca2+
40.08
Fluoride, F-
19.00
Nickel, Ni2+
58.69
Hydroxide, OH-
17.00
Potassium, K+
39.10
Nitrate, NO3-
62.00
Zinc, Zn2+
65.38
Sulfate, SO42-
96.00
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