CHEMISTRY 123-01 Midterm #1 – Answer key September 20, 2005 Statistics: · Average: 78 p (78%); · Highest: 100 p (100%); Lowest: 47 p (47%) · Number of students performing at or above average: 60 (60%) · Number of students with a score at or below 60 p (60%): 11 (11%) PART I: MULTIPLE CHOICE 1. (2 pts) All of the following properties of a substance can aid in its identification except: a. Density d. Temperature b. Melting point c. Boiling point 2. (2 pts) A sample has a mass of 294 g and a volume of 33 cm3. What is the identity of the sample? a. Mercury (density = 13.6 g/cm3) b. Copper (density = 8.92 g/cm3) c. Silver (density = 10.5 g/cm3) d. Aluminum (density = 2.7 g/cm3) e. Iron (density = 7.86 g/cm3) 3. (2 pts) Which of the following is a chemical property of iron? a. Iron melts at 1535 oC. b. Iron conducts heat. c. Iron can be bent into shapes. d. Iron rusts on exposure to water and oxygen. e. Iron conducts electricity. 4. (2 pts) Which of the following does not occur when ice melts? a. Its density changes. b. Its shape changes. c. Its molecular composition changes . d. The arrangement of molecules changes. e. The speed of the molecules changes. 5. (2 pts) Which of the following is not part of Dalton's Atomic Theory? a. An atom of one element can be chemically transformed into a different type of atom. b. Atoms of different elements combine in whole number quantities. c. Atoms of a given element have the same mass. d. Atoms of different elements have different masses. e. Matter is not created nor destroyed in a chemical reaction; the molecular arrangements are changed. 6. (2 pts) Which of the following represents a pair of allotropes? a. Chlorine and hydrogen chloride b. Oxygen and oxygen fluoride c. Methane and ethane d. Diamond and graphite e. Carbon and carbon tetrachloride 7. (2 pts) Which of the following is a metalloid? a. Helium (He, atomic number 2) b. Cerium (Ce, atomic number 58) c. Plutonium (Pu, atomic number 94) d. Sulfur (S, atomic number 16) e. Antimony (Sb, atomic number 51) 8. (2 pts) The Millikan oil drop experiment was used to determine a. The nuclear character of the atom b. Whether the atom was radioactive c. The electron distribution in the atom d. The atomic number of an atom e. The charge of an electron 9. (2 pts) Which metric prefix means 1 x 10-9? a. kilo d. micro b. nano e. milli c. pico 10. (3 pts) Which of the following statements is not true? a. 51V has an atomic number of 23. b. 95Mo contains 53 neutrons. c. 66Zn has the same number of electrons and protons. d. 56Fe has the same number of neutrons and protons. e. 12C has the same number of protons, neutrons and electrons. 11. (2 pts) Which statement best describes isotopes? a. Atoms with the same number of protons and different numbers of neutrons b. Atoms with the same number of neutrons and a different number of protons c. Atoms with the same numbers of protons and neutrons d. Atoms in the same horizontal period e. Atoms in the same vertical family 12. (2 pts) Which of the following statements concerning isotopes is true? a. Every element has 3 isotopes. b. All isotopes of a given element are radioactive. c. Various isotopes of the same element exhibit very different chemical reactivity. d. Various isotopes of the same element have the same mass. e. Various isotopes of the same element will have the same number of protons. 13. (2 pts) Which sample contains the greatest number of atoms (Think conceptually! You do not have to calculate the explicit number of atoms in each case!)? a. 50 g P b. 50 g Ca c. 50 g Ba 14. (2 pts) Which element can be classified as a transition metal? a. N d. Co b. Ne e. Sr c. Ce 15. (2 pts) Three elements that are likely to have similar chemical properties are: a. Sr, Se, Sb b. K, Ca, Sc c. Sc, Ti, V d. Ca, Sr, Ba e. H, Fe, F 16. (3 pts) Which formula – name combination is incorrect? a. CS2 carbon disulfide b. Cl2O7 chlorine heptaoxide c. P2O3 diphosphorus trioxide d. SbCl5 antimony pentachloride e. SCl2 sulfur dichloride 17. (3 pts) Find the correct combination of protons and electrons below for the oxide ion. a. 8 protons and 8 electrons b. 16 protons and 18 electrons c. 8 protons and 10 electrons d. 8 protons and 6 electrons e. 16 protons and 16 electrons 18. (2 pts) Give the formula for the ionic compound that forms between strontium and nitrogen. a. SrN d. SrN2 b. Sr2N e. Sr3N2 c. Sr3N 19. (2 pts) Which ionic compound is expected to have the highest melting point? a. NaCl c. AlN b. MgO d. KI 20. (3 pts) Which compound is incorrectly named? a. Ba(OH)2 barium hydroxide b. Na2SO4 sodium sulfite c. Fe(OH)3 iron(III) hydroxide d. NH4NO3 ammonium nitrate e. K2Cr2O7 potassium dichromate 21. (2 pts) In the reaction given below, if 10 moles of aluminum oxide are consumed, how many moles of oxygen gas are produced? 2 Al2O3 4Al + 3O2 a. 3 d. 12 b. 4 e. 15 c. 10 22. (2 pts) Classify the following reaction. 2 Al(s) + 3 CuSO4(aq) Al2(SO4)3(aq) + 3 Cu (s) a. combination c. displacement b. decomposition e. exchange 23. (2 pts) Classify the following reaction. CaCl2(aq) + H2SO4(aq) CaSO4(s) + 2 HCl (aq) a. combination e. exchange b. decomposition c. displacement 24. (2 pts) The Roman numerals in the reaction given represent the coefficients in the balanced chemical equation. What are the values of the coefficients? I CO + II NO III CO2 + IV N2 I II III IV a. 2 2 2 3 b. 1 1 1 2 c. 2 1 2 1 d. 1 2 2 1 e. 2 2 2 1 25. (2 pts) The Roman numerals in the reaction given represent the coefficients in the balanced chemical equation. What are the values of the coefficients? I K2S + II AlCl3 ?III KCl + IV Al2S3 I II III IV a. 1 1 2 1 b. 3 2 6 2 c. 3 2 6 1 d. 3 2 3 2 e. 2 3 2 3 PART II: SHORT ANSWER (Each short answer question has a 1-point value!!) 26. Oxygen (O2) and ozone (O3) are allotropes of the same element. 27. Ions that contain atoms of more than two elements are called polyatomic ions. 28. Carbon-containing compounds are known as organic compounds. 29. A compound that forms between a non-metal and a non-metal is a molecular compound. 30. Stoichiometric coefficients found in a balanced equation can be used to derive mole ratios. PART III: NOMENCLATURE OF MOLECULAR AND IONIC COMPOUNDS 31. (7 pts) Name the following compounds: a. Al2(SO4)3; e. Ba(ClO3)2 Aluminum sulfate Barium Chlorate b. Cl2O5; Dichlorine Pentaoxide f. ZnCO3; c. CBr4; Zinc(II) Carbonate Carbon Tetrabromide g. KNO3; d. Li2HPO4; Potassium Nitrate Lithium Hydrogen Phosphate 32. (6 pts) Write the correct molecular formula for each of the following compounds: a. Calcium Nitrite; d. Sodium Carbonate Decahydrate; Ca(NO2)2 Na2CO3.10H2O b. Copper(I) Cyanide; e. Potassium Permanganate; CuCN KMnO4 c. Magnesium Phosphate; f. Silver(I) Perchlorate Mg 3(PO4)2 AgClO4 37. (4 pts) Provide formulas for the following polyatomic cations or anions (Make sure to indicate the charge!): a. The ammonium cation; c. The periodate anion; + - NH4 IO4 b. The hydrogen carbonate anion; - d. The acetate anion HCO3 - CH3COO PART IV: CONCEPTS 38. The following diagram represents the reaction of A2 (gray spheres) with B2 (white spheres): a. (2 pts) Write a balanced chemical equation for the reaction; A2 + 3 B2 2 AB3 b. (2 pts) Identify the limiting reactant; Limiting reactant: B2 Reactant in excess: A2 c. (2 pts) How many moles of product can be made from 1.0 mol of A2 and 1.0 mol of B2? 1 mol A2 x 2 mol AB3/1 mol A2 = 2 mol AB3 1 mol B2 x 2 mol AB3/3 mol B2 = 2/3 mol AB3 The maximum # of moles that can be prepared from 1mol A2 and 1 mol B2 (theoretical yield) is 2/3 mol AB3 39. (3 pts) A hydrocarbon of unknown formula CxHy was submitted to combustion analysis with the following results (See diagram below). What is the empirical formula of the compound? CxHy + O2 = CO2 = H2O Solution: The product mixture contains 3 molecules of CO2 for each 4 molecules of H2O. This means that the starting material contains 3 atoms of C for each 8 atoms of H. The empirical formula is C3H8. PART V: CALCULATION PROBLEMS (Show your work in its entirety. Do not provide just a single number! Pay attention to significant figures!). 36. Potassium superoxide, KO2, is used in re-breathing gas masks to generate oxygen according to the equation: 4 KO2 (s) + H2O (l) KOH (s) + O2 (g) a. (2 pts) Balance the above equation; 4 KO2 (s) + 2 H2O (l) 4 KOH (s) + 3 O2 (g) b. (7 pts) If a reaction vessel contains 0.25 mol of KO2 and 0.15 mol of H2O, find the following: i. The limiting reactant in the process; Answer: Examine separately the amounts of a product generated from KO2 and H2O individually. Since the next question is about the quantity of O2, let us choose O2 as the product to study: 1) Mol amount of O2 from KO2: 0.25 mol KO2 x (3 mol O2/4 mol KO2) = 0.19 mol O2 2) Mol amount of O2 from H2O: 0.15 mol H2O x (3 mol O2/2 mol H2O) = 0.22 mol O2 Therefore KO2 is the limiting reactant.