March 5th Packet

2010 Ap@ CHEMISTRY FREE-RESPONSE QUESTIONS

CHEMISTRYSection II

(Total time-95 minutes)

Part ATime-55 minutes

YOU MAY USE YOUR CALCULATOR FOR PART A.

CLEARLY SHOW THE METHOD USED AND THE STEPS INVOLVED IN ARRIVING AT YOUR ANSWERS.It is to your advantage to do this, since you may obtain partial credit if you do and you will receive little or no credit ifyou do not. Attention should be paid to significant figures.

Be sure to write all your answers to the questions on the lined pages following each question in the booklet with thepink cover. Do NOT write your answers on the green insert.

Answer Questions 1,2, and 3. The Section II score weighting for each question is 20 percent.

1. Several reactions are carried out using AgBr, a cream-colored silver salt for which the value of the solubility-product constant, Ksp' is 5.0 x 10-13 at 298 K.

(a) Write the expression for the solubility-product constant, Ksp' of AgBr.

(b) Calculate the value of [Ag"] in 50.0 mL of a saturated solution of AgBr at 298 K.

(c) A 50.0 mL sample of distilled water is added to the solution described in part (b), which is in a beaker withsome solid AgBr at the bottom. The solution is stirred and equilibrium is reestablished. Some solid AgBrremains in the beaker. Is the value of [Ag+] greater than, less than, or equal to the value you calculated inpart (b) ? Justify your answer.

(d) Calculate the minimum volume of distilled water, in liters, necessary to completely dissolve a 5.0 g sampleof AgBr(s) at 298 K. (The molar mass of AgBr is 188 g molv.)

(e) A student mixes 10.0 mL of 1.5 x 10-4 M AgN03 with 2.0 mL of 5.0 x 10-4 M NaBr and stirs the resultingmixture. What will the student observe? Justify your answer with calculations.

(f) The color of another salt of silver, AgI(s), is yellow. A student adds a solution of NaI to a test tubecontaining a small amount of solid, cream-colored AgBr. After stirring the contents of the test tube, thestudent observes that the solid in the test tube changes color from cream to yellow.

(i) Write the chemical equation for the reaction that occurred in the test tube.

(ii) Which salt has the greater value of Ksp: AgBr or AgI ? Justify your answer.

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2010 Ap® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)

Answer Question 5 and Question 6. The Section II score weighting for these questions is 15 percent each.

Your responses to these questions will be scored on the basis of the accuracy and relevance of the information cited.Explanations should be clear and well organized. Examples and equations may be included in your responses whereappropriate. Specific answers are preferable to broad, diffuse responses. .

14

8

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12

10

pH6

4

2

oo 20.0 40.0 60.0 80.0

Volume of Titrant Added (rnl.)

5. A solution of 0.100 M HCI and a solution of 0.100 M NaOH are prepared. A 40.0 mL sample of one of thesolutions is added to a beaker and then titrated with the other solution. A pH electrode is used to obtain the datathat are plotted in the titration curve shown above.

(a) Identify the solution that was initially added to the beaker. Explain your reasoning.

(b) On the titration curve above, circle the point that corresponds to the equivalence point.

(c) At the equivalence point, how many moles of titrant have been added?

(d) The same titration is to be performed again, this time using an indicator. Use the information in the tablebelow to select the best indicator for the titration. Explain your choice.

Indicator pH Range ofColor Change

Methyl violet 0-1.6

Methyl red 4-6

Alizarin yellow 10-12

(e) What is the difference between the equivalence point of a titration and the end point of a titration?

(f) On the grid provided on the next page, sketch the titration curve that would result if the solutions in thebeaker and buret were reversed (i.e., if 40.0 rnL of the solution used in the buret in the previous titrationwere titrated with the solution that was in the beaker).



2010 Ap® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)

14.-----.-----.-----.-----~12+-----~-----4------~--~10+-----4-----4-----4-----~

8+-----4-----4-----~----~pH

6+-----4-----4-----4------

4+------+-----4------+------

2+------+----~------+------

0+-----+-----+-----4-----~o 20.0 40.0 60.0 80.0

Volume of Titrant Added CmL)


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2007 API!>CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)

Answer Question 5 and Question 6. The Section II score weighting for these questions is 15 percent each.

Your responses to these questions will be graded on the basis of the accuracy and relevance of the information cited.Explanations should be clear and well organized. Examples and equations may be included in your responses whereappropriate. Specific answers are preferable to broad, diffuse responses.

5. Answer the following questions about laboratory situations involving acids, bases, and buffer solutions.

(a) Lactic acid, HC3Hs03 ' reacts with water to produce an acidic solution. Shown below are thecomplete Lewis structures of the reactants.

HI

H :0: :0:I I 1\ ..

H-C-C-C-O-H +I IH H

..H-O: -

IH

In the space provided above, complete the equation by drawing the complete Lewis structures of thereaction products.

(b) Choosing from the chemicals and equipment listed below, describe how to prepare 100.00 mL of a 1.00 Maqueous solution of NH4Cl (molar mass 53.5 g mol-l). Include specific amounts and equipment whereappropriate.

NH4Cl(s)

Distilled water

50 mL buret

100 mL beaker

100 mL graduated cylinder100 mL volumetric flask

100 mL pipet

Balance

(c) Two buffer solutions, each containing acetic acid and sodium acetate, are prepared. A student adds 0.10 molof HCl to 1.0 L of each of these buffer solutions and to 1.0 L of distilled water. The table below shows thepH measurements made before and after the 0.10 mol of HCl is added.

pH Before pH AfterHCl Added HCI Added

Distilled water 7.0 1.0

Buffer 1 4.7 2.7

Buffer 2 4.7 4.3

(i) Write the balanced net-ionic equation for the reaction that takes place when the HCl is added tobuffer 1 or buffer 2.

(ii) Explain why the pH of buffer 1 is different from the pH of buffer 2 after 0.10 mol of HCl is added.

(iii) Explain why the pH of buffer 1 is the same as the pH of buffer 2 before 0.10 mol of HCI is added.

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2002 AP® CHEMISTRY FREE·RESPONSE QUESTIONS (Form 8)

8. The graph below shows the result of the titration of a 25 mL sample of a 0.10 M solution of a weak acid, HA,with a strong base, 0.10 M NaOH.

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V)

-~ /'

V --I-"

13

11

9pH

7

3

1o o 5 10 15 20 25

Milliliters of 0.10 M NaOH Added

30

(a) Describe two features of the graph above that identify HA as a weak acid.

(b) Describe one method by which the value of the acid-dissociation constant for HA can be determined usingthe graph above.

(c) On the graph above, sketch the titration curve that would result if 25 mL of 0.10 M Hel were used insteadof 0.10 M HA.

(d) A 25 mL sample of 0.10 M HA is titrated with 0.20 M NaOH.

(i) What volume of base must be added to reach the equivalence point?

(ii) The pH at the equivalence point for this titration is slightly higher than the pH at the equivalence pointin the titration using 0.10 M NaOH. Explain.

END OF EXAMINATION

Copyright © 2002 by College Entrance Examination Board. All rights reserved.Advanced Placement Program and AP are registered trademarks of the College Entrance Examination Board.

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March 5th Packet