Duration: 1½ Hours Total Marks: 100 INTERNAL EXAMINER : Dr H G Kruger EXTERNAL EXAMINER : Dr D Jaganyi University of Pietermaritzburg IMPORTANT : COMPLETE THIS SECTION IMMEDIATELY SURNAME AND INITIAL (optional) STUDENT NUMBER: SEAT NUMBER: SIGNATURE: This paper consists of 16 pages including a Periodic Table. Please ensure that you have them all. QUESTION INTERNAL EXTERNAL FINAL 1 (30) 2 (30) 3 (40) TOTAL (100) R = 8.314 JK -1 mol -1 = 0.0821 L atm K -1 mol -1 = 62.4 L Torr K -1 mol -1 1 atm = 101.325 kpa Vap. Press. of H 2 O at 26.0 o C = 25 Torr. PLEASE USE THE INFORMATION TECHNOLOGY DIVISION MCQ ANSWER SHEET FOR QUESTION 1
Page 2 QUESTION 1 PLEASE USE THE INFORMATION TECHNOLOGY DIVISION MCQ ANSWER SHEET FOR QUESTION 1 MULTIPLE CHOICE:. CHOOSE THE ONE ALTERNATIVE THAT BEST COMPLETES THE STATEMENT OR ANSWERS THE QUESTION. 1.1 A stock solution of HNO 3 is prepared and found to contain 13.5 M of HNO 3. If 25.0 ml of the stock solution is diluted to a final volume of 0.500 L, the concentration of the diluted solution is M. (a) 0.270 (b) 1.48 (c) 0.675 (d) 675 (e) 270 1.2 Pure acetic acid (HC 2 COOH) is a liquid and is known as glacial acetic acid. Calculate the molarity of a solution prepared by dissolving 10.00 ml of glacial acetic acid at 25 o C in sufficient water to give 500.0 ml of solution. The density of glacial acetic acid at 25 o C is 1.049 g/ml. (a) 1.26 x 10 3 (b) 21.0 (c) 0.0210 (d) 0.350 (e) 3.50 x 10-4 1.3 Silver ions can be precipated from aqueous solutions by the addition of aqueous chloride: Ag + (aq) + Cl (aq) AgCl (s) Silver chloride is virtually insoluble in water so that the reaction appears to go to completion. How many grams of solid NaCl must be added to 25.0 ml of 0.366 M AgNO 3 solution to completely precipitate the silver? (a) 9.15 x 10-3 (b) 1.57 x 10-4 (c) 0.535 (d) 0.157 (e) 6.39 x 10 3
Page 3 QUESTION 1 (continued) 1.4 A solution is prepared by adding 1.60 g of solid NaCl to 50.0 ml of 0.100 M CaCl 2. What is the molarity of chloride ion in the final solution? Assume that the volume of the final solution is 50.0 ml. (a) 0.747 (b) 0.647 (c) 0.132 (d) 0.232 (e) 0.547 1.5 The molecular weight of urea (NH 2 ) 2 CO), a compound used as a nitrogen fertilizer, is amu. (a) 44.0 (b) 43.0 (c) 60.1 (d) 8.0 (e) 32.0 1.6 What is the empirical formula of a compound that is 64.8% C, 13.6% H, and 21.6% O by mass? (a) C 4 HO 1 (b) C 5 HO 2 (c) C 8 H 20 O 2 (d) C 5 H 14 O 1 (e) C 4 H 10 O 1 1.7 Lithium and nitrogen react in a combination reaction to produce lithium nitride: 6Li (s) + N 2 (g) 2Li 3 N (s) How many moles of N 2 are needed to react with 0.500 mol of lithium? (a) 3.00 (b) 0.500 (c) 0.167 (d) 1.50 (e) 0.0833
Page 4 QUESTION 1 (continued) 1.8 Automotive air bags inflate when sodium azide decomposes explosively to its constituent elements: 2NaN 3 (s) 2Na (s) + 3N 2 (g) How many moles of N 2 are produced by the decomposition of 2.88 mol of sodium azide? (a) 1.92 (b) 8.64 (c) 4.32 (d) 0.960 (e) 1.44 1.9 Calcium hydride (CaH 2 ) reacts with water to form hydrogen gas: CaH 2 (s) + 2H 2 O (l) Ca(OH) 2 (aq) + 2H 2 (g) How many grams of CaH 2 are needed to generate 48.0 L of H 2 gas at a pressure of 0.888 atm and a temperature of 32 o C? (a) 50.7 (b) 0.851 (c) 143 (d) 35.8 (e) 71.7 1.10 Zinc reacts with a aqueous sulphuric acid to form hydrogen gas: Zn (s) + H 2 SO 4 (aq) ZnSO 4 (aq) + H 2 (g) In an experiment, 225 ml of wet H 2 is collected over water at 27 o C and a barometric pressure of 748 torr. How many grams of Zn have been consumed? The vapor pressure of water at 27 o C is 26.74 torr. (a) 4.79 x 10 6 (b) 0.567 (c) 567 (d) 431 (e) 4.31 x 10 5
Page 5 QUESTION 1 (continued) 1.11 The kinetic energy of a 7.3 kg steel ball travelling at 18.0 m/s J. (a) 1.2 x 10 3 (b) 66 (c) 2.4 x 10 3 (d)1.3 x 10 2 (e) 7.3 1.12 Which one of the following statements is false? (a) (b) (c) (d) (e) E is a state function. The actual numerical value of E can be measured. E = E final E initial E = q + w When a system undergoes a process in which it gains energy from the surroundings, the E for the process is positive. 1.13 The value of E for a system that performs 213 kj of work on its surroundings and loses 79 kj of heat is kj. (a) +292 (b) -292 (c) +134 (d) -134 (e) -213 1.14 Which one of the following is an exothermic process? (a) (b) (c) (d) (e) ice melting water evaporating boiling soup condensation of water vapor Ammonium thiocyanate and barium hydroxide are mixed at 25 o C: the temperature drops. 1.15 The value of H o for the reaction below is 72 kj. How many kj of heat is released when 1.0 mol of HBr is formed in this reaction? H 2 (g) + Br 2 (g) 2HBr(g) (a) 144 (b) 72 (c) 0.44 (d) 36 (e) -72 [30]
Page 6 QUESTION 2 2.1 How many grams of Ca(OH) 2 are needed to neutralize 25.0 ml of 0.100 M HNO 3? (3) 2.2 A sample of 70.5 mg of potassium phosphate is added to 15.0 ml of 0.050 M silver nitrate, resulting in the formation of a precipitate. The equation for the reaction is: K 3 PO 4 (ag) + 3 AgNO 3 Ag 3 PO 4 (s) + 3 KNO 3 (aq) (a) (b) What is the limiting reagent in the reaction? Calculate the theoretical yield, in grams, of the precipitate that forms. (4) (4)
Page 7 QUESTION 2 (continued) 2.3 H 2 and O 2 are mixed in a closed cylinder and then ignited. As the reaction occurs, the system loses 1150 J of heat to the surroundings. The reaction also causes the piston of the cylinder to rise due to an increase in volume. The rising piston does 480 J of work on the surroundings as it pushes a flying wheel. What is the change in the internal energy of the system?
Page 8 QUESTION 2 (continued) 2.4 Suppose we confine 1 g of butane and sufficient oxygen to completely combust it in a cylinder as above. The cylinder, however, is perfectly insulating, so that no heat can escape to the surroundings. A spark initiates combustion of the butane and CO 2 and H 2 O form. If we use this cylinder to measure the enthalpy change in the reaction, would the piston rise, fall, or stay the same? Motivate your answer. HINT: 2C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10 H 2 O(g) (3) 2.5 How much heat is released when 4.5 g of methane gas is burned in a constantpressure system? CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) H = 890 kj (3)
Page 9 QUESTION 2 (continued) 2.6 Propane is combusted in presence of oxygen to form CO 2 and H 2 O. Apply Hess s law on the following heats of formations to calculate the standard enthalpy change for the reaction where propane is burned in oxygen under standard conditions: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) 3 C(s) + 4H 2 (g) C 3 H 8 (g) H = 103.85 kj C(s) + O 2 (g) CO 2 (g) H = 393.5 kj H 2 O(l) ½H 2 (g) + ½O 2 (g) H = 285.8 kj (6) 2.7 Describe the process of converting Pyrite (FeS 2 ) to sulfuric acid. (5) [30]
Page 10 QUESTION 3 3.1 Write down a balanced reaction for Marble with hydrochloric acid. Indicate the phase of each molecule. 3.2 Write down the approximate composition of gun metal?
Page 11 QUESTION 3 (continued) 3.3 Use the electronegativity of the elements (given in brackets) to discuss the polarity of the LiH bond. Indicate the charges of each atom (use δ+ or δ only). Li (1.0) and H (2.1). (3) 3.4 Complete the following reactions (give an indication of the state of the products eg colour, solubility, gas, solid etc): H 2 SO 4 (aq) + R NH 2 (l) NH 3 (g) + HCl(g)
Page 12 QUESTION 3 (continued) Ca(OCl) 2 (g) + HCl(aq) Mg 2+ (aq) + NH 4 OH(aq) Ca 2+ (aq) + C 2 O 4 2 (aq) Ba 2+ (aq) + H 2 S(g) Ca 2+ (aq) + NH 4 OH(aq)
Page 13 QUESTION 3 (continued) Ca 2+ (aq) + NAOH (conc) Pb 2+ (aq) + NH 4 OH(aq) 3.5 Name 2 of the 4 main cement clinkers (give the approximate chemical composition as well as the industrial abbreviation). (4)
Page 14 QUESTION 3 (continued) 3.6 Explain how Germanium can be doped with Arsenic to produce a n-type semi conductor. (Give a 2-dimensional structure of the resulting crystal matrix). (5) 3.7 Define a primary explosive and give an example.
Page 15 QUESTION 3 (continued) 3.8 Discuss the rocket fuel used for the American space shuttles. (Give appropriate reactions). (4) [40]