2004 Ap CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)

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1 2004 Ap CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) Answer EITHER Question 2 below OR Question 3 printed on page 8. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 20 percent. 2. Answer the following questions related to hydrocarbons. (a) Determine the empirical formula of a hydrocarbon that contains 85.7 percent carbon by mass. (b) The density of the hydrocarbon in part (a) is 2.0 g L- i at 50 C and atm. (i) Calculate the molar mass of the hydrocarbon. (ii) Determine the molecular formula of the hydrocarbon. (c) Two flasks are connected by a stopcock as shown below. The 5.0 L flask contains CH 4 at a pressure of 3.0 atm, and the 1.0 L flask contains C 2 H 6 at a pressure of 0.55 atm. Calculate the total pressure of the system after the stopcock is opened. Assume that the temperature remains constant. 5.0 L 3.0 atm 1.0L 0.55 atm (d) Octane, CgH1g(l),has a density of g ml,"! at 20 C. A 255 ml sample of CgHig(l) measured at 20 C reacts completely with excess oxygen as represented by the equation below. Calculate the total number of moles of gaseous products formed. Copyright 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 7 GO ON TO THE NEXT PAGE.

2 2005 AP<!)CHEMISTRY FREE-RESPONSE QUESTIONS Answer EITHER Question 7 below OR Question 8 printed on page 14. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent. 7. Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. (a) At a pressure of 1 atm, the boiling point of NHi I) is 240 K, whereas the boiling point of NF 3 ( I) is 144 K. (i) Identify the intermolecular force(s) in each substance. (ii) Account for the difference in the boiling points of the substances. (b) The melting point of KCl(s) is 776 C, whereas the melting point of NaCl(s) is 801 C. (i) Identify the type of bonding in each substance. (ii) Account for the difference in the melting points of the substances. (c) As shown in the table below, the first ionization energies of Si, P, and CI show a trend. Element First Ionization Energy (kj mol ") Si 786 P 1,012 CI 1,251 (i) For each of the three elements, identify the quantum level (e.g., n = 1, n = 2, etc.) of the valence electrons in the atom. (ii) Explain the reasons for the trend in first ionization energies. (d) A certain element has two stable isotopes. The mass of one of the isotopes is amu and the mass of the other isotope is amu. (i) Identify the element. Justify your answer. (ii) Which isotope is more abundant? Justify your answer. Copyright 2005 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 13 GO ON TO THE NEXT PAGE.

3 2006 FREE-RESPONSE QUESTIONS (Form B) Answer EITHER Question 7 OR Question 8 below. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent. 7. Account for each of the following observations in terms of atomic theory and/or quantum theory. (a) Atomic size decreases from Na to CI in the periodic table. (b) Boron commonly forms molecules of the type BX 3. These molecules have a trigonal planar structure. (c) The first ionization energy of K is less than that of Na. (d) Each element displays a unique gas-phase emission spectrum. 8. Use chemical and physical principles to account for each of the following. (a) An aluminum container filled with an aqueous solution of CuS0 4 Include a chemical equation with your answer. eventually developed a leak. (b) The inside of a metal container was cleaned with steam and immediately sealed. Later, the container imploded. (c) Skin feels cooler after rubbing alcohol has been applied to it. (d) The redness and itching of the skin caused by ant bites (injections of methanoic acid, HC0 2 H) can be relieved by applying a paste made from water and baking soda (solid sodium hydrogen carbonate). Include a chemical equation with your answer. STOP END OF EXAM

4 2007 AP~ CHEMISTRY FREE-RESPONSE QUESTIONS 6. Answer the following questions, which pertain to binary compounds. (a) In the box provided below, draw a complete Lewis electron-dot diagram for the IF 3 molecule. (b) On the basis of the Lewis electron-dot diagram that you drew in part (a), predict the molecular geometry of the IF3 molecule. (c) In the S02 molecule, both ofthe bonds between sulfur and oxygen have the same length. Explain this observation, supporting your explanation by drawing in the box below a Lewis electron-dot diagram (or diagrams) for the S02 molecule. (d) On the basis of your Lewis electron-dot diagram(s) in part (c), identify the hybridization of the sulfur atom in the S02 molecule _ ~.- ~

5 2007 AP~ CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) First Second Third Ionization Energy Ionization Energy Ionization Energy (kj mol'") (kj mol-i) (kj mol") Element 1 1,251 2,300 3,820 Element ,560 6,910 Element ,450 7,730 Element 4 1,000 2,250 3, The table above shows the first three ionization energies for atoms of four elements from the third period of the periodic table. The elements are numbered randomly. Use the information in the table to answer the following questions. (a) Which element is most metallic in character? Explain your reasoning. (b) Identify element 3. Explain your reasoning. (c) Write the complete electron configuration for an atom of element 3. (d) What is the expected oxidation state for the most common ion of element 2? (e) What is the chemical symbol for element 2? (f) A neutral atom of which of the four elements has the smallest radius? STOP END OF EXAM

6 2009 CHEMISTRY FREE-RESPONSE QUESTIONS (d) A buffer solution is prepared by dissolving some solid NaOCI in a solution of HOCI at 298 K. The ph of the buffer solution is determined to be (i) Calculate the value of [H30+] in the buffer solution. (ii) Indicate which of HOCI(aq) or OCqaq) is present at the higher concentration in the buffer solution. Support your answer with a calculation. 2. A student was assigned the task of determining the molar mass of an unknown gas. The student measured the mass of a sealed 843 ml rigid flask that contained dry air. The student then flushed the flask with the unknown gas, resealed it, and measured the mass again. Both the air and the unknown gas were at 23.0 C and 750. torr. The data for the experiment are shown in the table below. Volume of sealed flask Mass of sealed flask and dry air Mass of sealed flask and unknown gas 843 ml g g (a) Calculate the mass, in grams, of the dry air that was in the sealed flask. (The density of dry air is 1.18 g VI at 23.0 C and 750. torr.) (b) Calculate the mass, in grams, of the sealed flask itself (i.e., if it had no air in it). (c) Calculate the mass, in grams, of the unknown gas that was added to the sealed flask. (d) Using the information above, calculate the value of the molar mass of the unknown gas. After the experiment was completed, the instructor informed the student that the unknown gas was carbon dioxide (44.0 g mol"). (e) Calculate the percent error in the value of the molar mass calculated in part (d). (f) For each of the following two possible occurrences, indicate whether it by itself could have been responsible for the error in the student's experimental result. You need not include any calculations with your answer. For each of the possible occurrences, justify your answer. Occurrence 1: The flask was incompletely flushed with CO 2 (g), in the flask. resulting in some dry air remaining Occurrence 2: The temperature of the air was 23.0 C, but the temperature of the CO 2 (g) was lower than the reported 23.0 C. (g) Describe the steps of a laboratory method that the student could use to verify that the volume of the rigid flask is 843 ml at 23.O c. You need not include any calculations with your answer The College Board. All rights reserved. Visit the College Board on the Web: GO ON TO THE NEXT PAGE.

7 2009 CHEMISTRY FREE-RESPONSE QUESTIONS CHEMISTRY PartB Time-40 minutes NO CALCULATORS MAY BE USED FOR PART B. Answer Question 4 below. The Section II score weighting for this question is 10 percent. 4. For each of the following three reactions, write a balanced equation in part (i) and answer the question in part (ii). In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You may use the empty space at the bottom of the next page for scratch work, but only equations that are written in the answer boxes provided will be graded. EXAMPLE: A strip of magnesium metal is added to a solution of silver(i) nitrate. (i) Balanced equation: M~ 1- '2- A-tt" ~ N1~2.~ + 2. ~ (ii) Which substance is oxidized in the reaction? ~~M~~.~~ ~~~ ~~-~~~ (a) A sample of solid iron(iii) oxide is reduced completely with solid carbon. I (i) Balanced equation, (ii) What is the oxidation number of carbon before the reaction, and what is the oxidation number of carbon after the reaction is complete?

8 2009 Ap CHEMISTRY FREE-RESPONSE QUESTIONS (b) Equal volumes of equimolar solutions of ammonia and hydrochloric acid are combined. I (i) Balanced equation, (ii) Indicate whether the resulting solution is acidic, basic, or neutral. Explain. (c) Solid mercury(ii) oxide decomposes as it is heated in an open test tube in a fume hood. I (i) Balanced equation, (ii) After the reaction is complete, is the mass of the material in the test tube greater than, less than, or equal to the mass of the original sample? Explain. YOU MAY USE THE SPACE BELOW FOR SCRATCH WORK, BUT ONLY EQUATIONS THAT ARE WRITTEN IN THE ANSWER BOXES PROVIDED WILL BE GRADED.

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