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AP®ChemistryCollege BoardExam QuestionsUnit 4: Chemical ReactionsStoichiometry

Stoichiometry (College Board AP® Chemistry): Exam Questions

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Stoichiometry

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1a1 mark

6 H+(aq) + 2 MnO4−(aq) + 5 H2C2O4 (aq) → 10 CO2 (g) + 8 H2O(l) + 2 Mn2+(aq)

A student dissolved a 0.139 g sample of oxalic acid, H2C2O4 , in water in an Erlenmeyer flask. Then the student titrated the H2C2O4 solution in the flask with a solution of KMnO4 , which has a dark purple color. The balanced chemical equation for the reaction that occurred during the titration is shown above.

Identify the species that was reduced in the titration reaction. Justify your answer in terms of oxidation numbers.

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1b1 mark

The student used a 50.0 mL buret to add the KMnO4(aq) to the H2C2O4(aq) until a faint lavender color was observed in the flask, an indication that the end point of the titration had been reached. The initial and final volume readings of the solution in the buret are shown below. Write down the initial reading and the final reading and use them to determine the volume of KMnO4(aq) that was added during the titration.

q7-june-2019---ap-chemistry

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1c1 mark

Given that the concentration of KMnO4(aq) was 0.0235 M, calculate the number of moles of MnO4− ions that completely reacted with the H2C2O4 .

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1d1 mark

The student proposes to perform another titration using a 0.139 g sample of H2C2O4 , but this time using 0.00143 M KMnO4(aq) in the buret. Would this titrant concentration be a reasonable choice to use if the student followed the same procedure and used the same equipment as before? Justify your response.

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2a1 mark

A manufacturer produces calcium carbonate (CaCO2) by reacting calcium hydroxide (Ca(OH)2) with carbon dioxide gas (CO2) according to the balanced equation below:

Ca(OH)2 (s) + CO2 (g) ⟶ CaCO3 (s) + H2O (l)

A sample of 185 g of calcium hydroxide reacts with excess carbon dioxide.

Calculate the number of moles of calcium hydroxide present in the sample.

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2b1 mark

Determine the mass of calcium carbonate produced from this reaction.

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2c2 marks

In a second trial, only 122 g of calcium carbonate were collected. Calculate the percent yield for the reaction and explain one possible reason why the yield is less than 100%.

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3a1 mark

A chemist reacts barium chloride solution with sodium sulfate solution to produce solid barium sulfate and aqueous sodium chloride, according to the balanced equation below:

BaCl2 (aq) + Na2SO4 (aq) ⟶ BaSO4 (s) + 2NaCl (aq)

A solution is prepared by mixing 75.0 mL of 0.250 M barium chloride with 60.0 mL of 0.350 M sodium sulfate.

Determine the limiting reactant.

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3b1 mark

Calculate the maximum mass of barium sulfate that can form.

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3c1 mark

A student filters the barium sulfate and dries it, finding the final mass of the solid to be 6.05 g. Calculate the percent yield.

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3d1 mark

Explain how using excess water during rinsing of the solid could affect the percent yield measurement.

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4a2 marks

A student performs a titration to determine the concentration of Fe2+ (aq)) using 0.0200 M MnO4- (aq). The balanced equation for the reaction is:

5Fe2+ (aq) + MnO4− (aq) + 8H+ (aq) ⟶ 5Fe3+ (aq) + Mn2+ (aq) + 4H2O (l)

In one trial, 25.00 mL of FeSO4 (aq) is titrated with 18.75 mL of 0.0200 M MnO4⁻ (aq).

i) Calculate the number of moles of MnO4⁻ (aq) added during the titration.

ii) Calculate the number of moles of FeSO4 (aq) that reacted.

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4b1 mark

Determine the concentration of the FeSO4 (aq) solution in mol/L.

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4c1 mark

The student repeats the experiment and obtains a significantly lower concentration of FeSO4 (aq). Suggest one procedural error that could account for this result.

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5a2 marks

A student analyzes an unknown hydrogencarbonate, XHCO3 (s). A 1.69 g sample of XHCO3 is dissolved in water to make 100.0 mL of solution. A 25.00 mL portion of this solution is titrated with 0.100 M HCl (aq). The balanced equation for the reaction is:

XHCO3 (aq) + HCl (aq) ⟶ XCl (aq) + CO2 (g) + H2O (l)

In one trial, 15.60 mL of 0.100 M HCl (aq) is required to react completely with the 25.00 mL sample of the XHCO3 solution.

i) Calculate the number of moles of HCl (aq) added during the titration.

ii) Calculate the number of moles of XHCO3 (aq) in the original 100.0 mL solution.

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5b2 marks

i) Determine the molar mass of XHCO3 (s) based on the 2.50 g sample.

ii) Identify the element X in XHCO3 (s).

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6a1 mark

A student investigates the thermal decomposition of sodium bicarbonate in a sealed flask at 398 K.

2NaHCO3 (s) → Na2CO3 (s) + H2O (g) + CO2 (g)

The student heats 8.40 g of NaHCO3 in an evacuated 2.00 L container. After the reaction reaches completion, the total pressure of the gaseous products is measured to be 1.22 atm at 398 K.

Calculate the number of moles of NaHCO3 placed in the container.

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6b1 mark

Determine the theoretical number of moles of gas produced from complete decomposition.

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6c2 marks

Use the ideal gas law to calculate the total number of moles of gas produced in the reaction.

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6d1 mark

Compare your answers to parts (b) and (c). Calculate the percent yield of the decomposition reaction.

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6e1 mark

Suggest one possible reason why the percent yield is less than 100%, even though all of the NaHCO3 was heated.

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6f1 mark

After the decomposition of Na2CO3, the solid residue remaining in the container is dissolved in water and reacted with excess HCl (aq).

Write a balanced molecular equation for the reaction that occurs between Na2CO3 and HCl.

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6g1 mark

The student bubbles the gas produced from part (f) through limewater, (Ca(OH)2 (aq)).

Describe the expected observation and identify the gas responsible for the change.

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6h2 marks

Instead of collecting the gas, the student titrates the resulting solution with silver nitrate (AgNO3).

Write the balanced net ionic equation for the reaction, and
explain how this titration could be used to confirm the amount of Na₂CO₃ present in the original sample.

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1a1 mark

A student is given a 25.0 mL sample of a solution of an unknown monoprotic acid and asked to determine the concentration of the acid by titration. The student uses a standardized solution of 0.110 M NaOH(aq), a buret, a flask, an appropriate indicator, and other laboratory equipment necessary for the titration.

The images below show the buret before the titration begins (below left) and at the end point (below right). What should the student record as the volume of NaOH(aq) delivered to the flask?

q7-june-2016---ap-chemistry

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1b1 mark

Based on the given information and your answer to part (a), determine the value of the concentration of the acid that should be recorded in the student's lab report.

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1c2 marks

In a second trial, the student accidentally added more NaOH(aq) to the flask than was needed to reach the end point, and then recorded the final volume. Would this error increase, decrease, or have no effect on the calculated acid concentration for the second trial? Justify your answer.

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2a1 mark

A student investigates the reaction between hydrochloric acid and calcium carbonate to produce carbon dioxide gas, water, and calcium chloride, as shown in the balanced equation below:

CaCO3 (s) + 2HCl (aq) ⟶ CaCl2 (aq) + CO2 (g) + H2O (l)

The student places a 4.15 g sample of calcium carbonate into excess hydrochloric acid and collects the carbon dioxide gas produced in a syringe at 298 K and 1.50 atm.

The student records the following data:

Time (s)

Volume of CO2 (mL)

0

0.0

20

84.5

40

158.0

60

217.3

80

251.6

100

2626.5

120

267.0

Calculate the maximum theoretical volume of carbon dioxide gas that can be produced from 3.80 g of CaCO3 at 25 oC and 101.3 kPa.

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2b2 marks

Using the axes provided below, plot a graph of volume of CO2 (y-axis) against time (x-axis) using the data in the table. Label any key features of the graph.

Blank graph paper with numbered axes; horizontal from 0 to 160, vertical from 0 to 280. No data plotted.

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2c1 mark

Based on the graph and data, justify whether the reaction went to completion and how the rate of reaction changed over time. Support your answer with evidence.

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3a2 marks

Sodium azide (NaN3) is produced for use in car airbags by reacting sodium amide (NaNH2) with dinitrogen monoxide gas (N2O) according to the balanced equation below:

2NaNH2 (s) + N2O (g) ⟶ NaN3 (s) + NaOH (aq) + NH3 (g)

A factory needs 600 g of sodium azide. The reaction has a 93.0% yield.

Calculate the theoretical mass of sodium amide required to produce 600.0 g of sodium azide.

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3b2 marks

Determine the actual mass of sodium amide that must be used.

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4a2 marks

A student analyzes an impure sample of an unknown carbonate compound with the formula X2CO3 (s). The sample has a mass of 2.90 g and is 85.0% X2CO3 by mass. The sample is dissolved in water to make 100.0 mL of solution. A 25.00 mL portion of this solution is titrated with 0.200 M HCl (aq). The balanced equation for the reaction is:

X2CO3 (aq) + 2HCl (aq) ⟶ 2XCl (aq) + CO2 (g) + H2O (l)

In one trial, 18.40 mL of 0.200 M HCl (aq) is required to react completely with the 25.00 mL sample of the solution.

i) Calculate the number of moles of HCl (aq) added during the titration.

ii) Calculate the number of moles of X2CO3 (aq) in the original solution.

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4b2 marks

Identify the element X in X2CO3 (s). Justify your answer.

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5a2 marks

A 0.2186 g sample of an unknown metal carbonate with the formula M2CO3 (s) containing 92.0% M2CO3 by mass is dissolved in water to make 250.0 mL of solution. A 25.00 mL portion of this solution is titrated with 0.0150 M HCl (aq). The balanced equation for the reaction is:

M2CO3 (aq) + 2HCl (aq) ⟶ 2MCl (aq) + CO2 (g) + H2O (l)

25.30 mL of 0.0150 M HCl (aq) is required to react completely with the 25.00 mL sample of the solution.

Calculate the number of moles of M2CO3 (aq) in the original 250.0 mL solution.

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5b1 mark

Identify the element M in M2CO3 (s). Justify your answer.

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5c1 mark

An excess volume of HCl (aq) was added past the endpoint during the titration. Explain how this error would affect the calculated molar mass of M2CO3.

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