CHEMISTRY 102A Spring 2012 Hour Exam II. 1. My answers for this Chemistry 102 exam should be graded with the answer sheet associated with:

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Betty Harmon
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1 . My answers for this Chemistry 0 exam should be graded with the answer sheet associated with: a) Form A b) Form B c) Form C d) Form D e) Form E. A sample of LSD (D-lysergic acid diethylamide, C 4 H 30 N 3 O, molar mass = g/mol) is added to sodium chloride (NaCl) to form a mixture. Given that a.00 gram sample of the mixture undergoes complete combustion to produce.0 grams of CO, what is the mass percentage of LSD in the mixture? The molar mass of CO is 44.0 g/mol. Assume the only carbon containing product is CO. a) 0.% b) % c) 38% d) 43% e) 59% 3. If.00 L of.00 M K S is mixed with.00 L of.00 M KHSO 4, what is the concentration of potassium ions in the final solution? a) M b).00 M c).33 M d).67 M e).00 M 4. A compound contains only carbon, hydrogen, and oxygen. Combustion of g of the compound produces.60 g CO and 0.80 g H O. What is the mass percent of hydrogen in the unknown compound? a) 6.98% H b) 4.95% H c).48% H d) 44.3% H e) 88.5% H 5. A compound contains only carbon, hydrogen, and oxygen. Combustion of g of the compound produces.60 g CO and 0.80 g H O. What is the empirical formula of the compound? a) C 7 H 6 O b) C H O c) C 3 H O 3 d) C 3 H 8 O e) C 3 H 3 O 6. A student added 50.0 ml of an NaOH solution to 00.0 ml of M HCl. The solution was then treated with an excess of aqueous Cr(NO 3 ) 3, resulting in formation of.06 g of precipitate. Calculate the concentration of the initial NaOH solution. The molar mass of the precipitate is 03.0 g/mol. a).0 M b).0 M c).4 M d). M e) 0.50 M 7. Which of the following substances should have the largest value of b in the van der Waals equation? a) He b) O c) CO d) H O e) Ne Page 3

2 8. When.00 L of.00 M Cu(NO 3 ) is added to.50 L of.00 M K 3 PO 4, a precipitate forms. Which of the following (a-d) is the correct net ionic equation for this precipitation reaction? a) Cu 4+ (aq) + PO 4 3 (aq) Cu 3 (PO 4 ) 4 (s) b) Cu + (aq) + PO 4 6 (aq) Cu 3 PO 4 (s) c) Cu + (aq) + PO 4 3 (aq) CuPO 4 (s) d) Cu 4+ (aq) + PO 4 6 (aq) Cu 6 (PO 4 ) 4 (s) e) None of the above (a-d) are the correct net ionic equation. 9. When.00 L of.00 M Cu(NO 3 ) is added to.50 L of.00 M K 3 PO 4, a precipitate forms. How many moles of precipitate can form? a).00 mol b).33 mol c).50 mol d).75 mol e).00 mol 0. Consider three identical flasks filled with different gases at the following conditions: Flask A: CO(g) at 760. torr and 0C Flask B: N (g) at 50. torr and 0C Flask C: H (g) at 00. torr and 0C Which flask contains the gas sample having the greatest average kinetic energy (per molecule) and which flask contains the gas sample having the greatest average velocity (per molecule)? a) The gas in flask A has the greatest average kinetic energy and the gas in flask B has the greatest average velocity. b) All gases in the various flasks have the same average kinetic energy and the gas in flask C has the greatest average velocity. c) The gas in flask C has the greatest average kinetic energy and all gases in the various flasks have the same average velocity. d) All gases in the various flasks have the same average kinetic energy and the gas in flask B has the greatest average velocity. e) All gases in the various flasks have the same average kinetic energy and the same average velocity.. A certain compound contains 49.0% carbon,.74% hydrogen and the rest chlorine by mass. What is the empirical formula of the compound? Page 4 a) C 3 H Cl b) C 3 H 3 Cl c) C HCl d) CHCl e) C 4 H 3 Cl

3 . A gas sample containing.50 moles at 5 C exerts a pressure of 400. torr. Some gas is added to the same container and the temperature is also increased to 50. C. If the pressure increases to 800. torr, how many moles of gas were added to the container? Assume a constant volume container. a) 3.5 mol b) 6.00 mol c).50 mol d).75 mol e).7 mol 3. Dinitrogen tetrafluoride, N F 4, and HF can be produced by the reaction of NH 3 with F as illustrated in the following unbalanced equation: NH 3 (g) + F (g) N F 4 (g) + HF(g) If 4.00 g of NH 3 and 4.0 g of F are allowed to react, what mass of HF can be produced? Molar mass NH 3 = 7.03 g/mol; molar mass F = g/mol; molar mass HF = 0.0 g/mol. a) 5.3 g HF b) 7.77 g HF c) 8.85 g HF d) 4. g HF e) 8.0 g HF 4. Some very effective rocket fuels are composed of lightweight liquids. The fuel composed of dimethylhydrazine [(CH 3 ) N H ] mixed with dinitrogen tetroxide was used to power the Lunar Lander in its missions to the moon. The two components react according to the following equation: (CH 3 ) N H (l) + N O 4 (l) 3 N (g) + 4 H O(g) + CO (g) If 50 g of dimethylhydrazine reacts with excess dinitrogen tetroxide and the product gases are collected at 7 C in an evacuated 50 L tank, what is the partial pressure of nitrogen gas produced and what is the total pressure in the tank assuming the reaction has 00% yield? The molar mass of dimethylhydrazine is 60. g/mol. a) 0.5 atm;.0 atm b) 0.5 atm;.3 atm c) 0.50 atm;.5 atm d) 0.50 atm;.9 atm e) 0.74 atm;. atm 5. Consider the following generic equation (which is assumed balanced): 3A + B C + D.0 mol A and.0 mol B react to form.0 mol C. What is the percent yield of this reaction? a).5% b) 5% c) 50.% d) 75% e) 00% Page 5

4 6. A 6.50 g sample of a diprotic acid (abbreviated H A in the balanced equation below) requires 37.5 ml of a M NaOH solution for complete reaction. Calculate the molar mass of the acid. H A(aq) + NaOH(aq) H O(l) + Na A(aq) a) 5.9 g/mol b) 3.5 g/mol c) 63.0 g/mol d) 03 g/mol e) 6 g/mol 7. Equal moles of N gas and O gas are mixed in a flexible reaction vessel and are then ignited to form gaseous N O. Assuming that the reaction goes to completion, what is the ratio of the final volume of the gas mixture to the initial volume of the gas mixture (V final :V initial =?) if both volumes are measured at the same temperature and pressure? (Hint: Nitrogen gas is the limiting reactant and don t forget to balance the equation.) a) 3:4 b) :3 c) 3: d) : e) : 8. Which of the following gases has the largest density at STP? a) He b) Ne c) Ar d) Kr e) All of the gases have the same density at STP. 9. Gas A has an effusion rate which is nine (9) times faster than that of gas B. Which of the following statements is true concerning the molar masses of gases A and B? a) Gas A has a molar mass that is 3 times heavier than gas B. b) Gas A has a molar mass that is 9 times heavier than gas B. c) Gas A has a molar mass that is 8 times heavier than gas B. d) Gas B has a molar mass that is 3 times heavier than gas A. e) Gas B has a molar mass that is 8 times heavier than gas A. 0. Which of the following statements is false? Page 6 a) Real gas molecules have a volume. b) Real gas molecules exert attractive forces on one another. c) Gases tend to behave most ideally when the container volume is small and when gas molecules have a relatively slow average velocity. d) As temperature increases, the average kinetic energy of a sample of gas molecules increases. e) At constant temperature, lighter gas molecules move at a faster average velocity than heavier gas molecules.

5 . Which of the following compounds has the highest vapor pressure at -00 C? a) NH 3 b) PH 3 c) AsH 3 d) SbH 3 e) H O. Which of the following statements (a-d) is true? a) In general, covalent intermolecular forces are stronger than the intermolecular forces in ionic compounds. b) Substances with very weak intermolecular forces are solids at room temperature. c) Intermolecular forces are much stronger than intramolecular forces (bonds). d) As temperature increases, the vapor pressure of a liquid increases. e) None of the above statements (a-d) are true. 3. Phosphorus can be prepared from calcium phosphate by the following reaction: Ca 3 (PO 4 ) (s) + 6 SiO (s) + 0 C(s) 6 CaSiO 3 (s) + P 4 (s) + 0 CO(g) Phosphorite is a mineral which contains Ca 3 (PO 4 ) plus other non-phosphorus containing compounds. What is the maximum amount of P 4 that can be produced from.0 kg of phosphorite if the phorphorite sample is 75% Ca 3 (PO 4 ) by mass? Assume an excess of the other reactants. Molar mass Ca 3 (PO 4 ) = 30.8 g/mol; molar mass P 4 = 3.88 g/mol. a) 0.75 kg b) 0.5 kg c) 0.50 kg d) 0.38 kg e) 0.0 kg 4. When Mg(s) reacts with HBr(aq), MgBr (aq) and H (g) are produced. What mass of magnesium is required to react completely with 0. g of HBr? a) 3.0 g b) g c) 6. g d) 0. g e) 0.3 g 5. The boiling points of the halogens increase when going from F to I. What type of intermolecular forces are responsible for this trend? a) dipole-dipole forces b) hydrogen bonding forces c) London dispersion forces d) electrostatic (ionic) forces Page 7

6 6. How many of the following five compounds are strong electrolytes? I. cobalt(ii) nitrate II. ammonium chromate III. hydrofluoric acid IV. strontium hydroxide V. magnesium sulfate a) b) c) 3 d) 4 e) 5 (All five of the above compounds are strong electrolytes.) 7. A sealed weather balloon containing 0.00 mol of He(g) rises to 5 km above the earth s surface. At this altitude, the volume of the balloon is 37.0 L at a pressure of 76.0 torr. What is the temperature of the earth s atmosphere at 5 km above the surface? a) 5 C b) 87 C c) 48 C d) 5 C e) C 8. Consider the products for the following four aqueous reactions: I. FeSO 4 (aq) + NaCl(aq) II. Pb(NO 3 ) (aq) + MgCl (aq) III. CaCl (aq) + (NH 4 ) SO 4 (aq) Which reactions result in the formation of a precipitate? a) II and III b) I, II and III c) II, III and IV d) I and III e) II and IV 9. How many of the following four statements is/are true? IV. K S(aq) + Ni(NO 3 ) (aq) I. A concentrated solution in water will always contain a strong or weak electrolyte. II. A strong electrolyte will break up into ions when dissolved in water. III. All covalent compounds are nonelectrolytes in water. IV. All ionic compounds are strong electrolytes in water. a) 0 (All are false.) b) c) d) 3 e) 4 (All are true.) 30. Consider the following pairs of compounds. Which compound in each pair has the higher boiling point? In CH O, carbon is the central atom. N vs CO H 3 C-CH 3 vs CH O HF vs KNO 3 a) CO; CH O; KNO 3 b) N ; CH O; KNO 3 c) N ; CH O; HF Page 8 d) CO; H 3 CCH 3 ; HF e) N ; H 3 CCH 3 ; KNO 3

7 USEFUL CONSTANTS/EQUATIONS K = C + 73 PV = nrt kg = 000 g R = L atm/k mol L = 000 ml Avogadro's number, N = atm = 760 torr = 760 mm Hg Mass % of A = rate rate mass of A(00) totalmass M (M molar mass) M M = Molarity = % yield = actual theoretical molsolute L solution P total = P + P + P STP = atm, 73 K (00) d = density = mass/volume PM = drt, M = molar mass P V n T PV n T Kinetic Energy = (/) mv KE AVE = (3/) RT, R = J/Kmol P measured an V (V measured nb) nrt Solubility rules:. Most nitrate salts are soluble.. Most salts of alkali metals and ammonium cations are soluble. 3. Most chloride, bromide, and iodide salts are soluble. Exceptions: salts containing Ag +, Pb +, and Hg + ions are insoluble. 4. Most sulfate salts are soluble. Exceptions: sulfates containing Ca +, Ba +, Pb +, and Hg + ions are insoluble. 5. Most hydroxide salts are insoluble. Exceptions: hydroxides containing alkali metals, Ba +, Sr +, and Ca + ions are soluble. 6. Most sulfide, carbonate, chromate, and phosphate salts are insoluble. Exceptions: salts of alkali metals and ammonium cations are soluble. Page 9

M = Molarity = mol solute L solution. PV = nrt % yield = actual yield x 100 theoretical yield. PM=dRT where d=density, M=molar mass

M = Molarity = mol solute L solution. PV = nrt % yield = actual yield x 100 theoretical yield. PM=dRT where d=density, M=molar mass Solubility Rules: 1. Most nitrate salts are soluble. 2. Most salts of alkali metals and ammonium cations are soluble. 3. Most chloride, bromide and iodide salts are soluble. Exceptions: salts containing