Stoichiometry (College Board AP® Chemistry)

Exam Questions

18 mins12 questions
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
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.

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

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.

2b1 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
2c1 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 .

2d1 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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