NAME: ___________________________________ AP Chemistry

NAME: ___________________________________
AP Chemistry
Unit 2 Test (Chapters 3 and 4)
1. A student is assigned the task of determining the mass percent of silver in an alloy of copper and silver by
dissolving a sample of the alloy in excess 16 M HNO3 nitric acid in a beaker and then precipitating the silver as
AgCl with a 0.2000 M NaCl solution prepared by the student.
a. During the dissolving of the alloy, the student accidentally spills about 2-3 mL of 16 M HNO3 on the
bench top. The student finds three bottles containing liquids sitting near the spill: a bottle of sodium
hydroxide, a bottle of sodium hydrogen carbonate and a bottle of sodium chloride. Which of the solids is
best to use in cleaning up the spill? Justify your choice. Include a balanced equation for the reaction that
takes place cleaning up the spill.
b. During the dissolving of the alloy describe any signs that a chemical reaction has taken place that indicate
that the alloy contained copper.
c. Using the ½ reaction method write a balanced chemical equation for the reaction between the copper and
the nitric acid.
d. The student then needs to prepare 250 mL of a 0.2000 M NaCl solution. The student is provided with a
stock bottle of solid sodium chloride, a 250 mL graduated cylinder that can be read to ±2 mL, a 250 mL
beaker that can be read to ±10 mL, a 250 mL volumetric flask that can be read to ±0.16 mL, an
analytical balance that can be read ±0.0001 g, an electronic balance that can be read ±0.1 g safety
goggles, a weighing dish, a wash bottle, and distilled H2O. Calculate the mass of sodium chloride needed
and briefly list the steps of an appropriate and safe procedure for preparing the 250 mL of a 0.2000 M
NaCl solution.
Initial Volume in buret
Volume in buret after first settling
Volume in buret after second settling
Volume in buret after third settling (no more precipitate forms)
2.46 mL
22.98 mL
24.48 mL
24.50mL
e. Then the student adds 16 M HNO3 into a beaker that has 0.6489 g sample of the alloy until all the alloy
dissolves. After the sample completely reacts with the acid, the alloy solution is transferred into an
Erlenmeyer flask and a titration is performed. The prepared sodium chloride solution is added using a
buret until no more precipitate forms after allowing the precipitate to settle to the bottom of the flask. The
data for that titration is in the table above.
i. Write a complete balanced equation for the reaction that takes place in the Erlenmeyer flask.
ii. Calculate the mass of the AgCl precipitate made.
iii. Calculate the mass percent of silver in the alloy of copper and silver.
iv. Explain why your percentage may or may not be exact.
2. Iron metal can be made from iron(III) oxide by reacting the iron(III) oxide with carbon monoxide with the other
product of this reaction being carbon dioxide. A 16.2 L sample of carbon monoxide at 1.50 atm and 200.oC is
combined with 15.39 g of iron(III) oxide.
a. Write a balanced chemical equation for the reaction. Circle the oxidizing agent and label it OA.
b. Calculate the moles of each reactant.
i. Carbon monoxide
ii. Iron(III) oxide
c. What is the limiting reactant for the reaction? Justify your answer with calculations.
d. How many grams of iron will be made?
A water solution of iron is found in the lab, but what is not know about the solution is what kind of iron ions it
contains, mainly iron (II) ions or iron (III) ions.
e. What visual observations could help in identifying the ion type?
f.
What substance or substances could be added to confirm this? Write a brief procedure of what you could
add and what visual observations you could use in your determination.
3. Answer the following questions that relate to the analysis of chemical compounds.
a. A compound containing the elements C, H, N, O is analyzed. When a 1.2359 g sample is burned in
excess oxygen gas, 2.241 g of CO2(g) is formed. The combustion analysis also showed the sample
contained 0.0648 g of H.
i. Determine the mass, in grams, of C in the 1.2359 g sample of the compound.
ii. When the compound is analyzed for N content only, the mass percent of N is found to be 28.44
percent. Determine the mass, in grams, of N in the original 1.2359 g sample of the compound.
iii. Determine the mass, in grams, of O in the original 1.2359 g sample of the compound.
iv. Determine the empirical formula of the compound.
b. A different compound, which has the empirical formula CH2Br, has a vapor density of 6.00 g L-1 at 375
K and 0.983 atm. Using these data, determine the following.
i. The molecular mass of the compound with appropriate units
ii. The molecular formula of the compound
4. Equation Section: Write the formulas to show the reactants and products for ALL of the laboratory situations described
below. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances
in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by
the reaction. You need to balance the equations, and then answer the follow-up question about the reaction.
Example: A strip of magnesium is added to a solution of silver nitrate.
+
2+
Mg + Ag  Mg + Ag
a. Bromine water is mixed through a solution of potassium iodide.
i. Balanced equation:
ii.
Who is the oxidizing agent?
b. Solid magnesium carbonate is added to a test tube containing acetic acid.
i. Balanced equation:
ii. How could you determine when the all the acid has reacted?
c. A solution of silver nitrate is added to a solution of sodium chloride.
a. What would you see when the reaction takes place inside a test tube?
d. Carbon dioxide gas is bubbled through water.
i. Balanced equation:
ii. What happens to the pH of the water?
e. Solid copper hydroxide is heated in test tube.
i. Balanced equation:
ii. What color changes would be observed during the reaction?
f.
A solution of chromium (III) chloride and then hydrogen peroxide are mixed in a basic solution.
i. Balanced equation:
ii. What color changes would be observed during this reaction?
g. Chlorine water and a solution of sodium sulfite are mixed in an acidified solution.
i. Balanced equation:
iii. What cation could be added to confirm the presence of the anions containing sulfur that is made during the
reaction?
5. Discuss the following phenomena in terms of the chemical and physical properties of the substances
involved and general principles of chemical and physical change.
a. What will be observed on the surfaces of zinc and silver strips shortly after they are placed in
separate solutions of CuSO4, as shown in the picture below. Account for these observations.
b. Chlorine water, Cl2 , in a basic solution and hydrogen peroxide, H2O2, in an acidic solution both
undergo a reaction that is called a disproportionate reaction. Using the products of each of these
two separate reactions to explain the definition of a disproportionate reaction.
6. Given solid samples of NaI and of AgNO3, briefly describe two simple laboratory tests by which these
two compounds can be distinguished. For each test, report the expected result for each compound.