The Advanced Placement Examination in Chemistry Part I Multiple Choice Questions Part II - Free Response Questions 1970 to 2010.

Transcription

1 The Advanced Placement Examination in Chemistry Part I Multiple Choice Questions Part II - Free Response Questions 1970 to 2010 Lab Procedures Part I 1984 Questions (A) CO 3 2 (B) Cr 2 O 7 2 (C) NH 4 + (D) Ba 2+ (E) Al 3+ Assume that you have an unknown consisting of an aqueous solution of a salt that contains one of the ions listed above. Which ion must be absent on the basis of each of the following observations of the unknown? 10. The solution is colorless 11. The solution gives no apparent reaction with dilute hydrochloric acid. 12. No odor can be detected when a sample of the solution is added drop by drop to a warm solution of sodium hydroxide. 13. No precipitate is formed when a dilute solution of H 2 SO 4 is added to a sample of the solution. 31. A 0.1 molar solution of which of the following ions is orange? (A) Fe(H 2 O) 4 2+ (B) Cu(NH 3 ) 4 2+ (C) Zn(OH) 4 (D) Zn(NH 3 ) 4 2+ (E) Cr 2 O The addition of an oxidizing agent such as chlorine water to a clear solution of an unknown compound results in the appearance of a brown color. When this solution is shaken with the organic solvent, methylene dichloride, the organic solvent layer turns purple. The unknown compound probably contains (A) K + (B) Br (C) NO 3 (D) I (E) Co Metallic copper is heated strongly with concentrated sulfuric acid. The products of this reaction are (A) CuSO 4 (s) and H 2 (g) only (B) Cu 2+, SO 2 (g), and H 2 O (C) Cu 2+, H 2 (g), and H 2 O (D) CuSO 4 (s), H 2 (g), and SO 2 (g) (E) Cu 2+, SO 3 (g), and H 2 O 61. When a solution of potassium dichromate is added to an acidified solution of iron(ii) sulfate, the products of the reaction are Page 1 of 17

2 (A) FeCr 2 O 7 (s) and H 2 O (B) FeCrO 4 (s) and H 2 O (C) Fe 3+, CrO 4 2, and H 2 O (D) Fe 3+, Cr 3+, and H 2 O (E) Fe 2 (SO 4 ) 3 (s), Cr 3+ and H 2 O 62. A student pipetted five milliliter samples of hydrochloric acid and transferred each sample to an Erlenmeyer flask, diluted it with distilled water, and added a few drops of phenolphthalein to each. Each sample was then titrated with a sodium hydroxide solution to the appearance of the first permanent faint pink color. The following results were obtained. Volumes of NaOH Solution First Sample ml Second Sample ml Third Sample ml Fourth Sample ml Fifth Sample ml Which of the following is the most probable explanation for the variation in the student s results? (A) The burette was not rinsed with NaOH solution (B) The student misread a 5 for a 6 on the burette when the first sample was titrated. (C) A different amount of water was added to the first sample. (D) The pipette was not rinsed with the HCI solution. (E) The student added too little indicator to the first sample. 72. A compound is heated to produce a gas whose molecular weight is to be determined. The gas is collected by displacing water in a water filled flask inverted in a trough of water. Which of the following is necessary to calculate the molecular weight of the gas, but does NOT need to be measured during the experiment? (A) Mass of the compound used in the experiment (B) Temperature of the water in the trough (C) Vapor pressure of the water (D) Barometric pressure (E) Volume of water displaced from the flask Appropriate laboratory procedures include which of the following? I. Rinsing a buret with distilled water just before filling it with the titrant for the first titration II. Lubricating glass tubing before inserting it into a stopper III. For accurate results, waiting until warm or hot objects have reached room temperature before weighing them (A) II only (B) I and II only (C) I and III only (D) II and III only (E) I, II, and III 44. The metal calcium reacts with molecular hydrogen to form a compound. All of the following statements concerning this compound are true EXCEPT: (A) Its formula is CaH 2. (B) It is ionic. Page 2 of 17

3 (C) It is solid at room temperature. (D) When added to water, it reacts to produce H 2 gas. (E) When added to water, it forms an acidic solution. 45. A measured mass of an unreactive metal was dropped into a small graduated cylinder half filled with water. The following measurements were made. Mass of metal = grams Volume of water before addition of metal = milliliters Volume of water after addition of metal = milliliters The density of the metal should be reported as (A) grams per ml (B) grams per ml (C) 7.84 grams per ml (D) 7.8 grams per ml (E) 8 grams per ml 63. Which of the following characteristics is common to elemental sulfur, chlorine, nitrogen, and carbon? (A) They are gaseous elements at room temperature. (B) They have oxides that are acid anhydrides. (C) They have perceptible color at room temperature. (D) They form ionic oxides. (E) They react readily with hydrogen at room temperature. 69. A white solid is observed to be insoluble in water, insoluble in excess ammonia solution, and soluble in dilute HCl. Which of the following compounds could the solid be? (A) CaCO 3 (B) BaSO 4 (C) Pb(NO 3 ) 2 (D) AgCl (E) Zn(OH) Adding water to some chemicals can be dangerous because large amounts of heat are liberated. Which of the following does NOT liberate heat when water is added to it? (A) KNO 3 (B) NaOH (C) CaO (D) H 2 SO 4 (E) Na When an aqueous solution of NaOH is added to an aqueous solution of potassium dichromate, K 2 Cr 2 O 7, the dichromate ion is converted to (A) CrO 4 2 (B) CrO 2 (C) Cr 3+ (D) Cr 2 O 3 (s) (E) Cr(OH) 3 (s) Page 3 of 17

4 38. Concentrations of colored substances are commonly measured by means of a spectrophotometer. Which of the following would ensure that correct values are obtained for the measured absorbance? I. There must be enough sample in the tube to cover the entire light path. II. The instrument must be periodically reset using a standard. III. The solution must be saturated. (A) I only (B) II only (C) I and II only (D) II and III only (E) I, II, and III 41. A strip of metallic scandium, Sc, is placed in a beaker containing concentrated nitric acid. A brown gas rapidly forms, the scandium disappears, and the resulting liquid is brown yellow but becomes colorless when warmed. These observations best support which of the following statements? (A) Nitric acid is a strong acid. (B) In solution scandium nitrate is yellow and scandium chloride is colorless. (C) Nitric acid reacts with metals to form hydrogen. (D) Scandium reacts with nitric acid to form a brown gas. (E) Scandium and nitric acid react in mole proportions of 1 to Mass of an empty container 3.0 grams Mass of the container plus the solid sample 25.0 grams Volume of the solid sample 11.0 cm 3 The data above were gathered in order to determine the density of an unknown solid. The density of the sample should be reported as (A) 0.5 g/cm 3 (B) 0.50 g/cm 3 (C) 2.0 g/cm 3 (D) 2.00 g/cm 3 (E) 2.27 g/cm Which of the following solids dissolves in water to form a colorless solution? (A) CrCl 3 (B) FeCl 3 (C) CoCl 2 (D) CuCl 2 (E) ZnCl When dilute nitric acid was added to a solution of one of the following chemicals, a gas was evolved. This gas turned a drop of limewater, Ca(OH) 2, cloudy, due to the formation of a white precipitate. The chemical was (A) household ammonia, NH 3 (B) baking soda, NaHCO 3 (C) table salt, NaCl (D) epsom salts, MgSO 4 7H 2 O (E) bleach, 5% NaOCl 56. It is suggested that SO 2 (molar mass = 64.1 grams), which contributes to acid rain, could be removed from a stream of waste gases by bubbling the gases through 0.25 molar KOH, thereby producing Page 4 of 17

5 K 2 SO 3. What is the maximum mass of SO 2 that could be removed by 1,000. liters of the KOH solution? (A) 4.0 kg (B) 8.0 kg (C) 16 kg (D) 20. kg (E) 40. kg 59. When a 1.00 gram sample of limestone was dissolved in acid, 0.38 gram of CO 2 was generated. If the rock contained no carbonate other than CaCO 3, what was the percent of CaCO 3 by mass in the limestone? (A) 17% (B) 51% (C) 64% (D) 86% (E) 100% 67. Substances X and Y that were in a solution were separated in the laboratory using the technique of fractional crystallization. This fractional crystallization is possible because substances X and Y have different (A) boiling points (B) melting points (C) densities (D) crystal colors (E) solubilities 69. Correct procedures for a titration include which of the following? I. Draining a pipet by touching the tip to the side of the container used for the titration II. Rinsing the buret with distilled water just before filling it with the liquid to be titrated III. Swirling the solution frequently during the titration (A) I only (B) II only (C) I and III only (D) II and III only (E) I, II, and III 70. To determine the molar mass of a solid monoprotic acid, a student titrated a weighed sample of the acid with standardized aqueous NaOH. Which of the following could explain why the student obtained a molar mass that was too large? I. Failure to rinse all acid from the weighing paper into the titration vessel II. Addition of more water than was needed to dissolve the acid III. Addition of some base beyond the equivalence point (A) I only (B) III only (C) I and II only (D) II and III only (E) I, II, and III The safest and most effective emergency procedure to treat an acid splash on skin is to do which of the following immediately? (A) Dry the affected area with paper towels Page 5 of 17

6 (B) Sprinkle the affected area with powdered Na 2 SO 4 (s) (C) Flush the affected area with water and then with a dilute NaOH solution (D) Flush the affected area with water and then with a dilute NaHCO 3 solution (E) Flush the affected area with water and then with a dilute vinegar solution 27. Appropriate uses of a visible-light spectrophotometer include which of the following? I. Determining the concentration of a solution of Cu(NO 3 ) 2 II. Measuring the conductivity of a solution of KMnO 4 III. Determining which ions are present in a solution that may contain Na +, Mg 2+, Al 3+ (A) I only (B) II only (C) III only (D) I and II only (E) I and III only 46. Which of the following occurs when excess concentrated NH 3 (aq) is mixed thoroughly with 0.1 M Cu(NO 3 ) 2 (aq)? (A) A dark red precipitate forms and settles out. (B) Separate layers of immiscible liquids form with a blue layer on top. (C) The color of the solution turns from light blue to dark blue. (D) Bubbles of ammonia gas form. (E) The ph of the solution decreases. 49. Which of the following techniques is most appropriate for the recovery of solid KNO 3 from an aqueous solution of KNO 3? (A) Paper chromatography (B) Filtration (C) Titration (D) Electrolysis (E) Evaporation to dryness 56. A yellow precipitate forms when 0.5 M NaI(aq) is added to a 0.5 M solution of which of the following ions? (A) Pb 2+ (aq) (B) Zn 2+ (aq) (C) CrO 4 2 (aq) (D) SO 4 2 (aq) (E) OH (aq) 65. Which of the following compounds is NOT appreciably soluble in water but is soluble in dilute hydrochloric acid? (A) Mg(OH) 2 (s) (B) (NH 4 ) 2 CO 3 (s) (C) CuSO 4 (s) (D) (NH 4 ) 2 SO 4 (s) (E) Sr(NO 3 ) 2 (s) 70. When 100 ml of 1.0 M Na 3 PO 4 is mixed with 100 ml of 1.0 M AgNO 3, a yellow precipitate forms and [Ag + ] becomes negligibly small. Which of the following is a correct listing of the ions remaining in solution in order of increasing concentration? (A) [PO 4 3 ] < [NO 3 ] < [Na + ] (B) [PO 4 3 ] < [Na + ] < [NO 3 ] Page 6 of 17

7 (C) [NO 3 ] < [PO 4 3 ] < [Na + ] (D) [Na + ] < [NO 3 ] < [PO 4 3 ] (E) [Na + ] < [PO 4 3 ] < [NO 3 ] 71. In a qualitative analysis for the presence of Pb 2+, Fe 2+, and Cu 2+ ions in aqueous solution, which of the following will allow the separation of Pb 2+ from the other ions at room temperature? (A) Adding dilute Na 2 S(aq) solution (B) Adding dilute HCl(aq) solution (C) Adding dilute NaOH(aq) solution (D) Adding dilute NH 3 (aq) solution (E) Adding dilute HNO 3 (aq) solution 72. After completing an experiment to determine gravimetrically the percentage of water in a hydrate, a student reported a value of 38 percent. The correct value for the percentage of water in the hydrate is 51 percent. Which of the following is the most likely explanation for this difference? (A) Strong initial heating caused some of the hydrate sample to spatter out of the crucible. (B) The dehydrated sample absorbed moisture after heating. (C) The amount of the hydrate sample used was too small. (D) The crucible was not heated to constant mass before use. (E) Excess heating caused the dehydrated sample to decompose Which of the following is a precipitation reaction? (A) H 2 SeO 4 (aq) + 2 Cl (aq) + 2 H + (aq) H 2 SeO 3 (aq) + Cl 2 (g) + H 2 O(l) (B) S 8 (s) + 8 O 2 (g) 8 SO 2 (g) (C) 3 Br 2 (aq) + 6 OH (aq) 5 Br (aq) + BrO 3 (aq) + 3 H 2 O(l) (D) Ca 2+ (aq) + SO 4 2 (aq) CuSO 4 (s) (E) PtCl 4 (s) + 2 Cl (aq) PtCl 6 2 (aq) 14. Which of the following is a combustion reaction? (A) H 2 SeO 4 (aq) + 2 Cl (aq) + 2 H + (aq) H 2 SeO 3 (aq) + Cl 2 (g) + H 2 O(l) (B) S 8 (s) + 8 O 2 (g) 8 SO 2 (g) (C) 3 Br 2 (aq) + 6 OH (aq) 5 Br (aq) + BrO 3 (aq) + 3 H 2 O(l) (D) Ca 2+ (aq) + SO 4 2 (aq) CuSO 4 (s) (E) PtCl 4 (s) + 2 Cl (aq) PtCl 6 2 (aq) 21. In the laboratory, H 2 (g) can be produced by adding which of the following to HCl(aq)? I. 1 M NH 3 (aq) II. Zn(s) III. NaHCO 3 (s) (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 32. Which of the following oxides is a gas at 25 C? (A) Rb 2 O (B) N 2 O (C) Na 2 O 2 (D) SiO 2 Page 7 of 17

8 (E) La 2 O A sample of a solution of an unknown was treated with dilute hydrochloric acid. The white precipitate formed was filtered and washed with hot water. A few drops of potassium iodide solution were added to the hot water filtrate and a bright yellow precipitate was produced. The white precipitate remaining on the filter paper was readily soluble in ammonia solution. What two ions could have been present in the unknown? (A) Ag + (aq) and Hg 2 2+ (aq) (B) Ag + (aq) and Pb 2+ (aq) (C) Ba 2+ (aq) and Ag + (aq) (D) Ba 2+ (aq) and Hg 2 2+ (aq) (E) Ba 2+ (aq) and Pb 2+ (aq) 50. Which of the following represents acceptable laboratory practice? (A) Placing hot objects on a balance pan (B) Using distilled water for the final rinse of a buret before filling it with standardized solution (C) Adding a weighed quantity of solid acid to a titration flask wet with distilled water (D) Using 10 ml of standard solution for titration of 25 ml of acid solution (E) Diluting a solution in a volumetric flask to its final concentration with hot water 65. Which of the following substances is LEAST soluble in water? (A) (NH 4 ) 2 SO 4 (B) KMnO 4 (C) BaCO 3 (D) Zn(NO 3 ) 2 (E) Na 3 PO A colorless solution is divided into three separate samples. The following tests were performed on samples of the solution. Sample Test Observation 1 Add H + (aq) No change 2 Add NH 3 (aq) No change 3 Add SO 2 4 (aq) No change Which of the following ions could be present in the solution at a concentration of 0.10 M? (A) Ni 2+ (aq) (B) Al 3+ (aq) (C) Ba 2+ (aq) (D) Na + (aq) (E) CO 3 2 (aq) 74. A pure, white crystalline solid dissolves in water to yield a basic solution that liberates a gas when excess acid is added to it. On the basis of this information, the solid could be (A) KNO 3 (B) K 2 CO 3 (C) KOH (D) KHSO 4 (E) KCl Which of the following is a precipitation reaction? (A) 2 Mg(s) + O 2 (g) 2 MgO(s) Page 8 of 17

9 (B) Pb 2+ (aq) + CrO 4 2 (aq) PbCrO 4 (s) (C) SO 3 (g) + 2 H 2 O(l) H 3 O + (aq) + HSO 4 (aq) (D) 2 H 2 O(g) 2 H 2 (g) + O 2 (g) (E) Ag + (aq) + 2 NH 3 (aq) [Ag(NH 3 ) 2 ] + (aq) Mass (g) Empty flask Flask + liquid The density of a pure liquid at 25 C was calculated by determining the mass and volume of a sample of the liquid. A student measured the mass of a clean, dry ml volumetric flask, filled the flask to its calibration mark with the liquid, and then measured the mass of the flask and liquid, and then measured the mass of the flask and liquid. The recorded measurements are shown in the table above. On the basis of this information, to how many significant figures should the density of the liquid be reported? (A) 3 (B) 4 (C) 5 (D) 6 (E) A student mixes equal volumes of 1.0 M solutions of tin(ii) chloride and copper(ii) sulfate and observes that no precipitate forms. Then the student mixes equal volumes of 1.0 M solutions of zinc(ii) sulfate and tin(ii) fluoride and observes the formation of a precipitate. The formula of the precipitate must be (A) SnF 2 (B) SnSO 4 (C) Sn(SO 4 ) 2 (D) ZnF (E) ZnF When diluting concentrated H 2 SO 4, one should slowly add acid to a beaker of water rather than add water to a beaker of acid. The reason for this precaution is to ensure that (A) there is complete ionization of the H 2 SO 4 (B) there is a sufficient volume of water to absorb the heat released (C) the water does not sink beneath the acid and remain unmixed (D) the acid does not react with impurities in the dry beaker (E) any SO 2 released quickly redissolves in the water 49. Salts containing which of the following ions are generally insoluble in cold water? (A) Acetate (B) Ammonium (C) Potassium (D) Nitrate (E) Phosphate 60. When aqueous NH 3 is first added to a solution containing Ni 2+, a precipitate forms, but when an excess of aqueous NH 3 is added, the precipitate dissolves. Which of the following best explains why the precipitate dissolves? (A) Ni 2+ forms hydrogen bonds with NH 3. (B) Ni 2+ forms a complex ion with NH 3. Page 9 of 17

10 (C) Ni 2+ acts as a Bronsted-Lowry base. (D) Ni 2+ is oxidized to Ni 3+. (E) Ni 2+ is reduced to Ni Which of the following pieces of laboratory glassware should be used to most accurately measure out a ml sample of a solution? (A) 5 ml pipet (B) 25 ml pipet (C) 25 ml beaker (D) 25 ml Erlenmeyer flask (E) 50 ml graduated cylinder Part II 1970 Zn Zn e E = 0.76 v Cu Cu e E = 0.34 v The solubility of H 2 S in water is approximately 0.1 molar. Zn(OH) 2 (s) + 2 OH 2 Zn(OH) 4 K = Cu(OH) 2 (s) + 2 OH 2 Cu(OH) 4 K = K sp of ZnS = 10 23, K sp of CuS = A solution is approximately 1 molar in Cu 2+ and 1 molar in Zn 2+. Based on the data above, outline three different methods for separating them discussing the theoretical bases for these separations A milliliter sample of NH 3 solution is titrated with a standard HCl solution. 1) An unknown volume of water is added to the HCl solution. 2) An unknown volume of water is added to the milliliter sample of NH 3 solution. 3) Phenolphthalein is used as the indicator. For each of these three steps taken during the titration: (a) State whether it introduces an error into the titration results. (b) For any of the steps that introduce(s) an error, state whether the titration result will be raised or lowered compared to the result obtained if the error had not been made. (c) Explain why the result is high or low for each error that you detect (1) Briefly describe four different laboratory tests by which NaNO 3 can be distinguished from NH 4 Cl (2) What minimum data are needed to determine the molecular weight of each of the following substances in the laboratory? In each case, use a different method and give the mathematical formula(s) to be used for calculating the molecular weight from these minimal data. (a) A liquid that is insoluble in water and that boils at 65 C (b) A solid nonelectrolyte 1974 The heat liberated when 1.00 mole of acetic acid, HC 2 H 3 O 2, reacts with 1.00 mole of sodium hydroxide, NaOH, is 12.7 kilocalories. Describe how this value can be determined in a general chemistry laboratory. Page 10 of 17

11 1975 Briefly outline a laboratory procedure that can be used to determine the composition of an alloy of copper and silver. The alloy dissolves completely in concentrated nitric acid In a laboratory determination of the atomic weight of tin, a sample of tin is weighed in a crucible. Nitric acid is added, and the reaction proceeds to give a hydrated tin(iv) oxide plus NO 2 and H 2 O. The hydrated tin(iv) oxide is then heated strongly and reacts as follows: SnO 2 xh 2 O(s) SnO 2 (s) + xh 2 O(g) The SnO 2 is finally cooled and weighed in the crucible. Explain the effect on the calculated atomic weight of tin that would result from each of the following experimental errors: (a) Considerable spattering occurs when the nitric acid is added to the tin. (b) The hydrated tin(iv) oxide is not heated sufficiently to change it completely to tin oxide Describe a laboratory procedure needed to carry out each of the following. (a) Separate a mixture of powdered solid CaCl 2 and CaCO 3. (b) Determine the concentration of solute in an aqueous sodium chloride solution and give the concentration units that your method provides. (c) Separate a mixture of two volatile liquids Given solid samples of KI and of (NH 4 ) 2 CO 3, briefly describe four simple laboratory tests by which these two compounds can be distinguished. For each test, report the expected result for each compound Describe a separate laboratory procedure for preparing each of the following. (a) Pure barium sulfate from an aqueous solution of barium chloride. (b) A pure aqueous solution of copper(ii) nitrate from solid copper(ii) carbonate. (c) A pure aqueous solution of calcium chloride from an aqueous solution of calcium bromide An experiment is to be performed to determine the standard molar enthalpy of neutralization of a strong acid by a strong base. Standard school laboratory equipment and a supply of standardized 1.00-molar HCl and standardized 1.00-molar NaOH are available. (a) What equipment would be needed? (b) What measurements should be taken? (c) Without performing calculations, describe how the resulting data should be used to obtain the standard molar enthalpy of neutralization. (d) When a class of students performed this experiment, the average of the results was kilojoules per mole. The accepted value for the standard molar enthalpy of neutralization of a strong acid by a strong base kilojoules per mole. Propose two likely sources of experimental error that could account for the result obtained by the class An experiment is performed to determine the empirical formula of a copper iodide formed by direct combination of elements. A clean strip of copper metal is weighed accurately. It is suspended in a test tube containing iodine vapor generated by heating solid iodine. A white compound forms on the strip of copper, coating it uniformly. The strip with the adhering compound is weighed. Finally, the compound is washed completely from the surface of the metal and the clean strip is dried and reweighed. Page 11 of 17

12 DATA TABLE Mass of clean copper strip Mass of copper strip and compound Mass of copper strip after washing grams grams grams (a) State how you would use the data above to determine each of the following. (Calculations not required.) (1) The number of moles of iodine that reacted (2) The number of moles of copper that reacted (b) Explain how you would determine the empirical formula for the copper iodide. (c) Explain how each of the following would affect the empirical formula that could be calculated. (1) Some unreacted iodine condensed on the strip. (2) A small amount of the white compound flaked off before weighing An experiment is to be performed to determine the molecular mass of a volatile liquid by the vapor density method. The equipment shown above is to be used for the experiment. A barometer is also available. (a) What data are needed to calculate the molecular mass of the liquid? (b) What procedures are needed to obtain these data? (c) List the calculations necessary to determine the molecular mass. (d) If the volatile liquid contains non-volatile impurities, how would the calculated value of the molecular mass be affected? Explain your reasoning Four bottles, each containing about 5 grams of finely powdered white substance, are found in a laboratory. Near the bottles are four labels specifying high purity and indicating that the substances are glucose (C 6 H 12 O 6 ), sodium chloride (NaCl), aluminum oxide (Al 2 O 3 ), and zinc sulfate (ZnSO 4 ). Assume that these labels belong to the bottles and that each bottle contains a single substance. Describe the tests that you could conduct to determine which label belongs to which bottle. Give the results you would expect for each test. Page 12 of 17

13 1996 A gram sample of a weak, nonvolatile acid, HA, was dissolved in sufficient water to make 50.0 milliliters of solution. The solution was then titrated with a standard NaOH solution. Predict how the calculated molar mass of HA would be affected (too high, too low, or not affected) by the following laboratory procedures. Explain each of your answers. (a) After rinsing the buret with distilled water, the buret is filled with the standard NaOH solution; the weak acid HA is titrated to its equivalence point. (b) Extra water is added to the gram sample of HA. (c) An indicator that changes color at ph 5 is used to signal the equivalence point. (d) An air bubble passes unnoticed through the tip of the buret during the titration An experiment is to be performed to determine the mass percent of sulfate in an unknown soluble sulfate salt. The equipment shown above is available for the experiment. A drying oven is also available. (a) Briefly list the steps needed to carry out this experiment. (b) What experimental data need to be collected to calculate the mass percent of sulfate in the unknown? (c) List the calculations necessary to determine the mass percent of sulfate in the unknown. (d) Would 0.20-molar MgCl 2 be an acceptable substitute for the BaCl 2 solution provided for this experiment? Explain An approximately 0.1-molar solution of NaOH is to be standardized by titration. Assume that the following materials are available. Clean, dry 50 ml buret 250 ml Erlenmeyer flask Wash bottle filled with distilled water Analytical balance Phenolphthalein indicator solution Potassium hydrogen phthalate, KHP, a pure solid monoprotic acid (to be used as the primary standard) (a) Briefly describe the steps you would take, using the materials listed above, to standardize the NaOH solution. Page 13 of 17

14 (b) Describe (i.e., set up) the calculations necessary to determine the concentration of the NaOH solution. (c) After the NaOH solution has been standardized, it is used to titrate a weak monoprotic acid, HX. The equivalence point is reached when 25.0 ml of NaOH solution has been added. In the space provided on your answer sheet, sketch the titration curve, showing the ph changes that occur as the volume of NaOH solution added increases from 0 to 35.0 ml. Clearly label the equivalence point on the curve. (d) Describe how the value of the acid-dissociation constant, K a, for the weak acid HX could be determined from the titration curve in part (c). (e) The graph below shows the results obtained by titrating a different weak acid, H 2 Y, with the standardized NaOH solution. Identify the negative ion that is present in the highest concentration at the point in the titration represented by the letter A on the curve below The molar mass of an unknown solid, which is nonvolatile and a nonelectrolyte, is to be determined by the freezing-point depression method. The pure solvent used in the experiment freezes at 10 C and has a known molal freezing-point depression constant, Kƒ. Assume that the following materials are also available. test tubes stirrer pipet stopwatch graph paper thermometer balance beaker ice hot-water bath (a) Using the two sets of axes provided on your answer sheet, sketch cooling curves for (i) the pure solvent and for (ii) the solution as each is cooled from 20 C to 0.0 C. (b) Information from these graphs may be used to determine the molar mass of the unknown solid. (i) Describe the measurements that must be made to determine the molar mass of the unknown solid by this method. (ii) Show the setup(s) for the calculation(s) that must be performed to determine the molar mass of the unknown solid from the experimental data. (iii) Explain how the difference(s) between the two graphs in part (a) can be used to obtain information needed to calculate the molar mass of the unknown solid. (c) Suppose that during the experiment a significant but unknown amount of solvent evaporates from the test tube. What effect would this have on (he calculated value of the molar mass of the solid (i.e., too large, too small, or no effect)? Justify your answer. (d) Show the setup for the calculation of the percentage error in a student s result if the student obtains a value of 126 g mol -1 for the molar mass of the solid when the actual value is 120. g mol -1. Page 14 of 17

15 2002 A student is asked to determine the molar enthalpy of neutralization, Hneut, for the reaction represented above. The student combines equal volumes of 1.0 M HCl and 1.0 M NaOH in an open polystyrene cup calorimeter. The heat released by the reaction is determined by using the equation q = mc T. Assume the following. Both solutions are at the same temperature before they are combined. The densities of all the solutions are the same as that of water. Any heat lost to the calorimeter or to the air is negligible. The specific heat capacity of the combined solutions is the same as that of water. (a) Give appropriate units for each of the terms in the equation q = mc T. (b) List the measurements that must be made in order to obtain the value of q. (c) Explain how to calculate each of the following. (i) The number of moles of water formed during the experiment (ii) The value of the molar enthalpy of neutralization, Hneut, for the reaction between HCl(aq) and NaOH(aq) (d) The student repeats the experiment with the same equal volumes as before, but this time uses 2.0 M HCl and 2.0 M NaOH. (i) Indicate whether the value of q increases, decreases, or stays the same when compared to the first experiment. Justify your prediction. (ii) Indicate whether the value of the molar enthalpy of neutralization, Hneut, increases, decreases, or stays the same when compared to the first experiment. Justify your prediction. (e) Suppose that a significant amount of heat were lost to the air during the experiment. What effect would this have on the calculated value of the molar enthalpy of neutralization, Hneut? Justify your answer A student is instructed to determine the concentration of a solution of CoCl 2 based on absorption of light (spectrometric/colorimetric method). The student is provided with a 0.10 M solution of CoCl 2 with which to prepare standard solutions with concentrations of M, M, M and M. (a) Describe the procedure for diluting the 0.10 M solutions to a concentration of M using distilled water, a 100 ml volumetric flask, and a pipet or buret. Include specific amounts where appropriate. The student takes the 0.10 M solution and determines the percent transmittance and the absorbance at various wavelengths. The two graphs below represent the data. Page 15 of 17

16 (b) Identify the optimum wavelength for the analysis. The student measures the absorbance of the M, M, M, M and 0.10 M solutions. The data are plotted below. (c) The absorbance of the unknown solution is What is the concentration of the solution? (d) Beer s Law is an expression that includes three factors that determine the amount of light that passes through a solution. Identify two of these factors. (e) The student handles the sample container (e.g., test tube or cuvette) that holds the unknown solution and leaves fingerprints in the path of the light beam. How will this affect the calculated concentration of the unknown? Explain your answer. (f) Why is this method of determining the concentration of CoCl 2 solution appropriate, whereas using the same method for measuring the concentration of NaCl solution would not be appropriate? 2004 In a laboratory class, a student is given three flasks that are labeled Q, R, and S. Each flask contains one of the following solutions: 1.0 M Pb(NO3)2, 1.0 M NaCl, or 1.0 M K2CO3. The student is also given two flasks that are labeled X and Y. One of these flasks contains 1.0 M AgNO3, and the other contains 1.0 M BaCl2. This information is summarized in the diagram below. (a) When the student combined a sample of the solution Q with a sample of X, a precipitate formed. A precipitate also formed when samples of solutions Q and Y are combined. (i) Identify solution Q. (ii) Write the chemical formulas for each of the two precipitates. (b) When solution Q is mixed with solution R, a precipitate forms. However, no precipitate forms when solution Q is mixed with solution S. (i) Identify solution R and solution S. (ii) Write the chemical formula of the precipitate that forms when solution Q is mixed with solution R. (c) The identity of solution X and solution Y are to be determined using the following solutions: 1.0 M Pb(NO3)2, 1.0 M NaCl, and 1.0 M K2CO3. Page 16 of 17

17 (i) Describe a procedure to identify solution X and solution Y. (ii) Describe the observations that would allow you to distinguish between solution X and solution Y. (iii) Explain how the observations would enable you to distinguish between solution X and solution Y Answer the following questions that relate to laboratory observations and procedures.. (a) An unknown gas is one of three possible gases: nitrogen, hydrogen, or oxygen. For each of the three possibilities, describe the result when the gas is tested using a glowing splint (a wooden stick with one end that has been ignited and extinguished, but still contains hot, glowing, partially burned wood). (b) The following three mixtures have been prepared: CaO plus water, SiO2 plus water, and CO2 plus water. For each mixture, predict whether the ph is less than 7, equal to 7, or greater than 7. Justify your answers. (c) Each of three beakers contains a 0.1 M solution of one of the following solutes: potassium chloride, silver nitrate, or sodium sulfide. Three beakers are labeled randomly as solution 1, solution 2, and solution 3. Shown below is a partially completed table of observations made of the results of combining small amounts of different pairs of the solutions. Solution 1 Solution 2 Solution 3 Solution 1 black precipitate Solution 2 no reaction Solution 3 (i) Write the chemical formula of the black precipitate. (ii) Describe the expected results of mixing solution 1 with solution 3. (iii) Identify each of the solutions 1, 2 and 3. Page 17 of 17