THE COLLEGE OF THE BAHAMAS

Transcription

1 THE COLLEGE OF THE BAHAMAS FINAL EXAMINATION KEY SEMESTER FACULTY OF PURE AND APPLIED SCIENCES SCHOOL OF CHEMISTRY, ENVIRONMENTAL & LIFE SCIENCES X NASSAU FREEPORT EXUMA ELEUTHERA DATE AND TIME OF EXAMINATION: Monday, April 19, 2010, 2 pm DURATION: 3 HOURS COURSE NUMBER: CHEM 135 COURSE TITLE: College Chemistry I SECTION NUMBER: (Circle your section number.) STUDENT NAME: STUDENT NUMBER: LECTURER S NAME INSTRUCTIONS TO CANDIDATES: This paper has 11 pages and 3 sections. Please follow the instructions given with each section.

2 CHEMISTRY 135 SEMESTER FINAL EXAMINATION CONTD. PERIODIC TABLE OF THE ELEMENTS Useful Information Molar volume of any gas at s.t.p. is 22.4 dm 3 mol -1 K -1 R = 8.31 J mo -1 K -1 = L mol -1 K -1 = 62.4 L Torr mol -1 K -1 Kinetic theory equation is 1 2 PV = nm u 3 Avogadro s constant = mol -1 1 atm = 760 Torr = Pa I II Groups III IV V VI VII H He hydrogen helium Li Be B C N O F Ne lithium beryllium boron carbon nitrogen oxygen fluorine neon Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium paladium silver cadmium indium tin antimony tellurium iodine xenon Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn cesium barium lanthanum hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon Fr 88 Ra 89 Ac francium radium actinium Lanthanides: Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium Actinides: Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr thorium protoactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium Note: relative atomic masses are omitted for highly unstable elements. PAGE 2 OF 11 6 May 10

3 SECTION A: MULTIPLE CHOICE QUESTIONS INSTRUCTIONS: Answer ALL questions in this section on the ANSWER SHEET PROVIDED using a soft pencil. There is one mark for each question in this section. Questions 1) to 5) are concerned with the shapes of molecules and ions. The following terms may be used to describe the shapes of molecules or ions. A linear B trigonal planar C tetrahedral D trigonal bipyramid E octahedral Select, from A to E, the term that best describes the shape of the following. Each term may be used once, more than once, or not at all. 1) CF 4 C 2) BeF 2 A 3) BF 3 B 4) CO 3 2- B 5) SCl 6 E *************************************** 6) Which one of the following molecules is most polar? A F 2 B FCl C CH 4 D FBr E H 2 7) The maximum number of electrons that a single orbital can hold is: A 2 B 3 C 6 D 8 E 10 8) Identical volumes of three triatomic gases CO 2, NH 3 and N 2 O, measured at the same temperature and pressure, were placed in a container at s.t.p. Which of the following statements is most accurate? A Only P = P = P is true. CO2 NH 3 N2O 1 B Only P TOT = P N2 O is true 3 C # of moles of CO 2 = 1/3 # of moles of N 2 O D Both A and B are true. E None of the above is correct. 9) Under which of the following conditions would you expect hydrogen gas to behave most like an ideal gas? A 273 K and 1.00 atm B 573 K and 3.00 atm C 753 K and 0.50 atm D 0 K and 3.00 atm E 100 K and 1.00 atm 10) The temperature of a gas decreases at fixed pressure. Which of the following statements is the most accurate? A Molecular mass decreases. B Average molecular speed decreases. C Pressure decreases. D Density decreases. E Volume increases. 11) Two atoms were found, by experimental determination, to have the same number of protons and electrons but different masses. Which of the following best accounts for this observation? A The atoms are from elements that are close together in the periodic table. B One of the atoms is in combination with another atom. C The atoms are of the same element but they have different numbers of neutrons. D The atoms are of the same element, but have different numbers of protons. E This cannot be explained in the ways described above. 12) When a group I element, X, combines with a group VI element, Y, A X atoms each lose one electron and Y atoms each gain 1 electron. B Y atoms each gain two electrons. C One X atom shares two electrons with two Y atoms. D X atoms lose one electron and Y atoms each gain two electrons. E Covalent bonds are formed. 13) Sulphur dioxide gas may best be identified by its ability to: A relight a glowing splint. B turn limewater milky. C turn red litmus paper blue. D dissolve in warm distilled water E change the colour of acidified potassium dichromate paper 14) What is the empirical formula for the compound whose molecular formula is C 6 H 10 O 2? A C 6 H 10 O 2 B C 3 H 5 O C CHO D C 12 H 20 O 4 PAGE 3 OF11

4 E None of the above 15) Neon has a simplified electron configuration of 2,8. Which of the following atoms or ions below is isoelectronic with the neon atom? A Na + B Ca 2+ C F D O E Cl 16) A sample of ozone gas has a volume of 44.8 dm 3 at s.t.p. How many moles of gas are present? A B 2.0 C 4 D 44.8 E ) Select the substance for which the standard enthalpy of formation is zero. A O 2- (g) B NH 3 (g) C N 2 (g) D Fe 2 O 3 (s) E CaO(s) 18) Which one of the following species has an electronic configuration which may be represented by [Ar]3d 3 4s 0. (Relevant atomic numbers are given at the beginning of this section.) A Fe 2+. B Iron as the free element. C Mn 2+. D Vanadium as the free element E Cr ) Which one of the following species has three unpaired electrons in the 2p sub-shell? A a hydrogen atom. B an oxygen atom. C a boron atom. D a nitrogen atom. E a phosphorus atom. 20) Hydrogen chloride has a lower boiling point than water partly because A it is a covalent compound. B its molecules have less tendency to form hydrogen bonds. C it has a larger molecular mass. D it does not contain ionic bonds. E it contains carbon atoms. 21) X moles of sodium chlorate(v) decomposed on heating to release 120 cm 3 of oxygen. 2NaClO 3 (s) 2NaCl(s) + 3O 2 (g) How many moles of sodium chlorate decomposed? (The molar volume of a gas under the same conditions is 24 dm 3 mol -1.) A B C D E ) Which one of the following molecules has a dipole moment? A I 2 B O 2 C N 2 D F 2 E HF 23) Which one of the following samples of gas occupies the greatest volume at s.t.p.? Note that the RMM of CO 2 = 44, Ne = 20, N 2 = 28, C 2 H 6 = 30, O 2 = 32. A 1 g of CO 2 B 1g of N 2 C 1g of O 2 D 1g of Ne E 1g of C 2 H 6 24) 34.2g of aluminium sulphate, Al 2 (SO 4 ) 3, (RFM 342) were dissolved in water and made up to 1 dm 3. What was the molarity of the solution with respect to sulphate ions? A 0.03 B 0.3 C 0.15 D 0.1 E ) Which one of the following elements has a first ionization energy higher than that of carbon? A silicon B beryllium C chlorine D sodium E magnesium 26) The Pauli exclusion principle tells us that A When orbitals have the same energy one of them is filled completely before any is occupied singly. B All electrons in an atom have the same values of their four quantum numbers. C An electron in an s-orbital has an l-value of 0. D No orbital may contain more than two electrons. E All the electrons in an atom are present in one orbital. PAGE 4 OF11

5 27) Sulfur and oxygen form a compound in which 3.2 g of sulfur are combined with 4.8 g of oxygen. (Relative atomic masses: S = 32, O = 16). What is the empirical formula of the compound? A S 3 O B SO C SO 3 D S 2 O E SO 2 28) Which one of the following molecules possesses a V -shape? A BeCl 2 B H 2 O C NH 3 D HF E CO 2 29) Given the RAM of Ca = 40, C = 12, O = 16, it may be said that calcium carbonate contains A 16 g of oxygen. B 1 mol of carbon. C 40 g of calcium. D ⅓ mol of calcium atoms for every mole of oxygen atoms. E 3 g of oxygen for every gram of calcium. 30) Which one of the following is NOT a basic assumption of the kinetic theory of gases? A The atoms or molecules of a gas are in random motion. B The kinetic energy of the molecules in a gas increases as the temperature rises. C Collisions between the molecules are perfectly elastic. D The molecules have a random distribution of speeds. E The molecules have a measurable size. 31) Which one of the following is a possible explanation for the deviation of real gases from ideal behaviour? A The molecules move with different speeds. B There is a force of attraction between the molecules. C The molecules have appreciable mass. D The molecules frequently collide with each other. E The molecules frequently collide with the walls of the containing vessel. 32) The molar masses of neon and argon are 20 g mol -1 and 40 g mol -1 respectively. Which one of the following statements is true at the same temperature and pressure? The density of Ar A = 2 *** The density of Ne The rate of diffusion of Ar B = 2 The rate of diffusion of Ne C The average kinetic energy of Ar molecules is higher than that of Ne molecules. D The mass of one molecule of Ne = The mass of one molecule of Ar E Equal masses of the two gases occupy the same volume. 33) Which one of the following statements about 1 mol of sodium carbonate is true? (Na =23 C = 12, O = 16, L represents Avogadro's number) A It contains 1 sodium ion. B It contains 2 sodium ions. C It contains L sodium ions. D It contains 2L carbonate ions. E It contains 3L ions altogether. 34) 300 cm 3 of a 2 00 M solution of KCl is mixed with 300 cm 3 of a 3 00 M KCl solution. The concentration of the resulting solution is A 5 00 M B 3 00 M C 2 50 M D 2 40 M E 2 00 M 35) Element X has an electronic structure 1s 2 2s 2 2p 6 3s 2 3p 1. Element Y has an electronic structure 1s 2 2s 2 2p 5. The simplest formula of a compound formed between X and Y is most likely to be A X 3 Y 5 B X 5 Y C XY D XY 3 E X 3 Y 36) Which one of the following has the electronic structure of argon? A Fe 2+ B Cu 2+ C Al 3+ D F - E Ca 2+ 37) What volume of oxygen would be required for the complete combustion of 100 cm 3 of hydrogen? A 50 cm 3 B 100 cm 3 C 200 cm 3 PAGE 5 OF11

6 D 22.4 cm 3 E 22.4 dm 3 38) In which one of the following pairs is the radius of the second atom greater than that of the first atom? A K, Ca B N, P C I, Br D Si, C E S, O 39) The volume of water which must be added to 150 cm 3 a 0.5 M solution of NaCl to make it 0.25 M, is A 50 cm 3 B 100 cm 3 C 150 cm 3 D 200 cm 3 E 300 cm 3 40) Which one of the following elements in the second period has the most negative value of first electron affinity? A Boron B Carbon C Nitrogen D Fluorine E Neon 41) Which one of the following atoms normally contains no unpaired electrons? A Lithium B Beryllium C Boron D Carbon E Oxygen 42) Which one of the following atoms has the highest ionisation energy? A Sodium B Magnesium C Aluminium D Silicon E Phosphorus 43) An electron in an atom has principle quantum number n = 3. How many different values of the quantum number l are possible for it? A 1 B 2 C 3 D 4 E 5 PAGE 6 OF11

7 SECTION B: STRUCTURED QUESTIONS INSTRUCTIONS: Answer ALL questions in this section in the spaces provided in the question paper. In numerical questions working must be shown, answers must be clearly indicated with the correct number of significant figures or decimal places. 1) Qualitative analysis and net ionic equations Three salt solutions were tested in an effort to identify the salts present. The results are presented in the table below. Use the results to identify the ions as completely as you can by filling in the spaces in the table with ONE possibility in each case. (6) TEST Sample A Sample B Sample C Add sodium hydroxide solution dropwise Add barium nitrate solution followed by dilute nitric acid Add silver nitrate solution White ppt. forms which is soluble in excess hydroxide solution No change observed. Creamy ppt. forms which is insoluble in dilute ammonia solution but dissolves in concentrated ammonia solution No change observed A dense white ppt. forms which dissolves in nitric acid to release a gas which turns limewater cloudy Test not necessary. Red-brown ppt. forms which is insoluble in excess hydroxide solution A dense white ppt. forms which is insoluble in nitric acid Test not necessary. Flame test Not carried out Lilac flame Not carried out CATION in sample Zn 2+, Al 3+, Pb 2+ Li +, Na +, K +, Ca 2+ Fe 3+, (Ag + ) ANION in sample Br - CO 3 2-, HCO 3 -, SO 3 2- a) Write net ionic equations for ALL the chemical reactions described in the table above. (11) Sample A: 4 equations Zn 2+ (aq) + 2OH - (aq) Zn(OH 2 (s) Zn(OH) 2 (s) + 2OH - (aq) [Zn(OH) 4 ] 2- (aq) Ag + (aq) + Br - (aq) AgBr(s) AgBr(s) + 2NH 3 (aq) [Ag(NH 3 ) 2 ] + (aq) + Br - (aq) Sample B: 3 equations Ba 2+ (aq) + CO 3 2- BaCO 3 (s) BaCO 3 (s) + 2H + (aq) Ba 2+ (aq) + H 2 O(l) + CO 2 (g) CO 2 (g) + Ca 2+ (aq) + 2OH - (aq) CaCO 3 (s) + H 2 O(l) Sample C: 2 equations Fe 3+ (aq) + 3OH - (aq) Fe(OH) 3 (s) Ba 2+ (aq) + SO 4 2- (aq) BaSO 4 (s) b) Give a possible chemical formula for any ONE of the three salts. (1) Many possibilites SO 4 2- PAGE 7 OF11

8 2) The kinetic theory and gas behaviour a) Calculate the pressure that mol of helium gas would exert if confined to a 4.48 dm 3 container at 25 C. (3) P = (nrt)/v = ( [273+25])/4.48 = atm to 3 s.f. b) Oxygen, nitrogen and hydrogen chloride were placed in a 10.0 dm 3 container so that the ratio of the number of moles was 1.00 : 4.00 : The total pressure in the container at was 1.00 atm. Calculate the partial pressure of nitrogen in the container. (3) P n (1) P P N 2 TOT P O 2 4 = = P 10 TOT = atm (2) MP c) i) From the ideal gas equation, derive the relationship D =, where D is the density of the gas and RT M is its molar mass. (5) PV = nrt n P = V RT m But n = m M P = (1 mark for each logical step) VM RT m But D = V D P = M RT MP D = RT ii) Calculate the density of O 2 at 125 C and 2.00 atm pressure. (2) MP D = = = g L to 3 s. f. RT ( ) PAGE 8 OF11

9 3) Atomic structure and bonding a) Give the detailed electronic configuration, in the form 1s 2 2s 2 2p x 1 etc, for each of the following species:(3) i) S 1s 2 2s 2 2p 6 3s 2 3p x 2 3p y 1 3p z 1 ii) Al 3+ 1s 2 2s 2 2p 6 iii) Fe 3+ 1s 2 2s 2 2p 6 3s 2 3p 6 4s 0 3d 5 b) Describe, in general terms, how first ionization energy (I 1 ) varies across a period left to right, and down a group. (2) I 1 increases across a period and decreases down a group. c) Explain the term polar covalent bond. (3) A polar covalent bond is one in which the electrons are not shared equally (1), resulting in a slight positive charge on the less electronegative atom (1) and a slight negative charge on the more electronegative atom (1). d) Give Lewis structures for each of the following molecules. Show ALL valence electrons and any formal charges. (4) i) H 2 O ii) CO 2 iii) CO (2 marks, 1 for electrons, 1 for charges) 4) Stoichiometry a) Calculate the molarity of a solution prepared by dissolving 26.1 g of barium nitrate (Ba(NO 3 ) 2 ) in PAGE 9 OF11

10 enough distilled water to give a final volume of exactly 2500 cm 3? (3) Molar mass of Ba(NO 3 ) 2 = ( ) =261.3 g mol -1 (1) Therefore number of moles of Ba(NO 3 ) 2 = (26.1 g)/(261.3 g mol -1 ) = mol (1) Therefore molarity = ( mol)/(2500 cm 3 ) (1000 cm 3 )/(1 dm 3 ) = mol dm mol dm -3 to 3 s.f. (1) b) 8.45 g of magnesium metal was allowed to react with excess hydrochloric acid. The equation is: Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) Calculate the mass of HCl that would be consumed in this reaction. (5) Molar mass of Mg = 24.3 g mol -1 ( ½) Molar mass of HCl = = 36.5 g mol -1 ( ½) Therefore 8.45 g Mg 8.45 g Mg (1 mol Mg / 24.3 g Mg) (1) (2 mol HCl / 1 mol Mg) (1) (36.5 g HCl / 1 mol HCl) (1) = g HCl to 3 s.f.(1) SECTION C: Essay questions There are three questions in this section. Select any ONE and answer it on the lined paper provided. 1) g of pure oxalic acid, (CO 2 H) 2.2H 2 O, was weighed out accurately and transferred to a 250 cm 3 volumetric flask without loss. The acid was dissolved and made up to the mark with water cm 3 portions were accurately pipetted into a conical flask and titrated with the potassium permanganate solution. The mean of three concordant titres was cm 3. Calculate the molarity of the potassium permanganate solution. 5(CO 2 H) 2 (aq) + 2MnO 4 - (aq) + 6H + (aq) 8H 2 O(l) + 2Mn 2+ (l) + 10CO 2 (l) (5) Explain why no indicator is required. (2) Why was the reaction mixture heated? (1) Molar mass of oxalic acid = 2 ( ) + 2 ( ) = g mol -1 (1) # of mol of oxalic acid = g (1mol/126g) = mol (1) Therefore # of mol in 1 aliquot = mol (1) mol C 2 O 4 H 2 (2 mol MnO 4 - / 5 mol C 2 O 4 H 2 ) = mol Mno 4 - (1) Therefore molarity of MnO 4 - = ( mol / 26 cm 3 ) (1000 cm 3 / 1 dm 3 ) = mol dm -3 (1) mol dm -3 to 4 s.f. (1) No indicator is required since MnO 4 - is highly coloured (1) so the solution in the conical flask becomes coloured when the equivalence point is passed. (1) The reaction mixture has to be heated so as to speed up the reaction, which is otherwise too slow at room temperature to give a clear-cut end-point. (1) 2) The volume of a sample of tetrachloromethane (CCl 4 ) in the vapour state is 22.5 cm 3 at a pressure of torr and a temperature of 20 C. a) Calculate the number of moles of tetrachloromethane present. (3) b) Calculate the mass of the tetrachloromethane. (2) c) Calculate the volume of the tetrachloromethane as a liquid at 20 C, given that the density of liquid tetrachloromethane at 20 C is 1.594g cm -3. (1) d) The tetrachloromethane was adsorbed on a solid surface as a continuous film only one molecule in thickness. Under these conditions it covered an area of 300 cm 2. Using your result from (c), estimate the thickness of the film and hence the diameter of a tetrachloromethane molecule, assuming that it is spherical. (2) a) PV = nrt and so n = (PV)/(RT) (1) = ( /1000)/[62.4 (273+20)] (1) = mol = mol to 3 s.f. (1) b) Molar mass of CCl 4 = = 154 g mol -1 Therefore mass of CCl 4 = mol 154 g mol -1 = g g to 3 s.f. PAGE 10 OF11

11 c) Volume of CCl 4 (l) g (1 cm 3 /1.594 g) = cm cm 3 to 3 s.f. d) Thickness = (volume)/(area) = cm 3 / 300 cm 2 = cm. Therefore diameter of a molecule cm 3) Using relevant bond energy values from the table below, estimate the enthalpy change involved in the reaction of gaseous ethanol to form carbon dioxide and water vapour according to the equation: C 2 H 5 OH(g) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(g) given: (8) Bond Bond energy (kj mol -1 ) Bond Bond energy (kj mol -1 ) C-C 348 C=O 799 C-H 413 O=O 495 C-O 358 O-H 463 Bonds broken (3) H Bonds formed (2) H 5 C-H = C=O C-C O-H C-O O-H O=O Total kj mol -1 Total kj mol -1 Note: H for bonds formed is negative. Therefore H r = (-5974) = kj mol -1 PAGE 11 OF11