School of Chemistry and Physics Westville Campus, Durban MODEL ANSWER

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1 School of Chemistry and Physics Westville Campus, Durban BARCODE Test 2: Wednesday 9 April 2014 CHEM110: GENERAL PRINCIPLES OF CHEMISTRY Duration: 45 minutes Total Marks: 25 Time: 17:45 18:30 Examiner: M. Shozi IMPORTANT: Complete this part immediately. Name MODEL ANSWER Student No. Tutorial Group Tutorial Venue Tutor s Name INSTRUCTIONS: 1. Answer ALL questions. 2. Calculators may be used but all workings must be shown. 3. The pages of this paper must not be unpinned. 4. Your answers must be written on the question paper in the spaces provided for section B ONLY. The left hand facing pages may be used for extra space or for rough work. 5. Marks will be deducted for the incorrect use of significant figures and the omission of units. 6. You must write legibly in black or blue ink. Pencils and Tipp-Ex TM are not allowed. 7. This question paper consists of 8 pages excluding a periodic table and data sheet. Please check that you have them all. 8. A periodic table and a data sheet are provided. Section A Total Section B Total Test Total Maximum Mark Mark

2 SECTION A - Multiple Choice Questions For each of the following questions, select the correct answer from the list provided. There is only one correct answer for each question. Indicate your answer on the multiple choice answer sheet provided. Make a dark heavy mark with HB pencil that fills the block of the appropriate letter completely. 1. What is the oxidation number of sulfur in HSO 4? A +2 B +4 C +6 D +8 (1) 2. Identify the reducing agent in the following reaction: A Cu Cu + 4 HNO 3 Cu(NO 3 ) NO H 2 O B HNO 3 C Cu(NO 3 ) 2 D NO 2 (1) 3. Which of the following is a weak base? A NaOH B NH 3 C CsOH D Ca(OH) 2 (1) Page 2 of 8

3 4. A disproportionation reaction is one in which A the equation is not balanced. B one reactant is in greater proportion than the other. C the same reactant is oxidised and reduced. D water is added. (1) 5. What volume of M HCl is needed to prepare ml of M HCl? A 1660 ml B ml C ml D ml (1) 6. What is the concentration of OH ions in a L solution of M Ca(OH) 2? A M B M C M D M (2) Page 3 of 8

4 7. What volume of M SO 3 2 is needed to titrate ml of M Fe 3+? 2 Fe 3+ (aq) + SO 3 2 (aq) + H 2 O(l) 2 Fe 2+ (aq) + SO 4 2 (aq) + 2 H + (aq) A ml B ml C ml D 6.00 ml (2) 8. Boyle s Law states that for a fixed amount of gas at constant temperature, A its pressure increases as its volume increases. B its pressure increases as its volume decreases. C its pressure is equal to its volume. D its pressure increases and its volume remains the same. (1) 9. One mole of O 2 (g) A has a mass of g. B occupies about the same volume as a mole of Cl 2 (g) at STP. C occupies L at any conditions of temperature and pressure. D contains two atoms. (1) Page 4 of 8

5 10. Ideal behaviour for a gas is most likely to be observed under conditions of A low temperature and high pressure. B low temperature and low pressure. C high temperature and low pressure. D high temperature and high pressure. (1) 11. What is the density (in g L -1 ) of N 2 gas at 350 K and atm? A 1.11 B 8.87 x 10-3 C D 113 (2) [14] Page 5 of 8

6 SECTION B: Full Questions. provided. Answer on the question paper in the space 1. Balance the following redox reaction in basic medium using the half-reaction method. Show all relevant steps. Cr(OH) 3 + ClO CrO Cl (4) Oxidation half-reaction: 2 H 2 O + Cr(OH) 3 CrO H + + 3e- (x2) (½ each for balancing O, H, charge) Reduction half-reaction: 2e- + 2 H + + ClO _ Cl + H 2 O (x3) (½ each for balancing O, H, charge) Overall reaction: 2 Cr(OH) ClO _ 2 CrO Cl + 4 H + + H 2 O 4 OH + 2 Cr(OH) ClO _ 2 CrO Cl + 4 H + + H 2 O + 4 OH 4 OH + 2 Cr(OH) ClO _ 2 CrO Cl + 5 H 2 O (½ each for adding 4 OH and for overall balanced reaction) Page 6 of 8

7 2. To determine the purity of acetylsalicylic acid (C 9 H 8 O 4, g mol -1 ) in an aspirin tablet, a g sample of aspirin was reacted with ml of M NaOH and the solution was heated to complete the reaction as shown below: C 9 H 8 O 4 (s) + 2 NaOH(aq) CH 3 COONa(aq) + HOC 6 H 4 COONa(aq) + H 2 O(l) After the reaction mixture was cooled, the excess NaOH was back-titrated with ml of M HCl according to the following reaction: NaOH(aq) + HCl(aq) NaCl(aq) + H 2 O(l) What is the percentage purity (% m/m) of acetylsalicylic acid in the aspirin sample? (4) Moles of NaOH added = L x M = mol ½ Moles of HCl used = L x M = mol ½ Therefore moles excess NaOH = moles HCl used = mol ½ Moles NaOH reacted with acetylsalicylic acid = mol = mol 1 Moles of acetylsalicylic acid = ½ x moles NaOH = mol ½ Mass acetylsalicylic acid = mol x g mol -1 = g ½ % purity = ( g g) x 100 = % ½ Page 7 of 8

8 3. Ammonium nitrite (NH 4 NO 2, g mol -1 ) decomposes on heating to form nitrogen gas and water: NH 4 NO 2 (s) N 2 (g) + 2 H 2 O(l) Decomposition of a sample of NH 4 NO 2 produces ml of N 2 gas collected over water at K and atm. Calculate the mass of NH 4 NO 2 which decomposed. Vapour pressure of H 2 O at K is atm. (3) From Dalton s law of partial pressures:.: P N2 = P Total - P H2O = = atm ½ From Ideal gas law: 1½ (½ for equation and 1 for calculation) i) Using R = L atm K -1 mol -1 PV atm x L n N2 = = RT L atm K -1 mol -1 x K = x 10-2 mol ii) Using R = kpa dm 3 K -1 mol -1 PV kpa x dm 3 n N2 = = = x 10-2 mol RT kpa dm 3 K -1 mol -1 x K Moles NH 4 NO 2 = moles N 2 = x 10-2 mol ½ Mass NH 4 NO 2 =n x MM= (1.989 x 10-2 mol)(64.05 g mol -1 ) = g ½ [11] END OF PAPER Page 8 of 8