AP CHEM WKST KEY: Sem 2 Review Part 5
(1) The unknown compound is white. In the table below, cross out the two compounds that can be eliminated using this
observation. Be sure to cross out these compounds in the remaining tables as well.
Al(NO3)3•9H2O
BaCl2•2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2•6H2O
2+
Note: Cu solutions are blue. Ni
will have a color other than white.
2+
CuSO4•5H2O
solutions are green. Majority of the time, solutions containing a transition metal
(2) When the unknown compound is added to water, it dissolves readily. In the table below, cross out the two
compounds that can be eliminated using this test. Be sure to cross out these compounds in the remaining tables as
well.
Al(NO3)3•9H2O
BaCl2•2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2•6H2O
CuSO4•5H2O
Note: Carbonate compounds are insoluble unless with an alkali metal or ammonium. BaSO 4 is insoluble even though
many sulfates are soluble—you found this out last year in Chemistry.
(3) When AgNO3(aq) is added to an aqueous solution of the unknown compound, a white precipitate forms. In the table
below, cross out each compound that can be eliminated using this test. Be sure to cross out the same compound(s)
in the remaining tables.
Al(NO3)3•9H2O
BaCl2•2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2•6H2O
CuSO4•5H2O
+
Note: The nitrate in Al(NO3)3 will not precipitate with Ag . However, you know from the “chloride test” last year that
+
−
Ag will precipitate with Cl .
(4) When the unknown compound is carefully heated, it loses mass. In the table below, cross out each compound that
can be eliminated using this test.
Al(NO3)3•9H2O
BaCl2•2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2•6H2O
CuSO4•5H2O
Note: Hydrates will lose mass when heated because the water is driven off by the heat.
(5) Describe a test that can be used to confirm the identity of the unknown compound identified in part (4). Limit your
confirmation test to a reaction between an aqueous solution of the unknown compound and an aqueous solution of
one of the other soluble salts listed in the tables. Describe the expected results of the test; include formula(s) of any
product(s).
By adding some CuSO4 solution to the unknown, a white precipitate (BaSO4) will form. The other soluble substances
will not precipitate a product when added to BaCl2 since they either contain nitrate or chloride which are soluble with
2+
−
Ba or a cation that is soluble with Cl .
BaCl2(aq) + CuSO4(aq) → BaSO4(s) + CuCl2(aq)