PART I: MULTIPLE CHOICE (30 multiple choice questions. Each multiple choice question is worth 2 points)
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1 CHEMISTRY Midterm #1 Answer key October 14, 2010 Statistics: Average: 74 p (74%); Highest: 97 p (95%); Lowest: 33 p (33%) Number of students performing at or above average: 67 (57%) Number of students performing below 55%: 17 (15%) Number of students at or above 90%: 12 (10%) PART I: MULTIPLE CHOICE (30 multiple choice questions. Each multiple choice question is worth 2 points) 1. All of the following properties of a substance can aid in its identification except: a. Density b. Melting point 2. Which of the following is a chemical property of iron? a. Iron melts at 1535 o C. b. Iron conducts heat. c. Iron can be bent into shapes. d. Iron rusts on exposure to water and oxygen. e. Iron conducts electricity. c. Boiling point d. Temperature 3. 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. 4. Which of the following represents a pair of allotropes? a. Chlorine and hydrogen chloride b. Oxygen and oxygen fluoride c. Water and ammonia d. Oxygen and ozone e. Carbon and carbon tetrachloride 5. 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 6. Which of the following statements is not true? a. 51 V has an atomic number of 23. b. 95 Mo contains 53 neutrons. c. 66 Zn has the same number of electrons and protons. d. 56 Fe has the same number of neutrons and protons. e. 12 C has the same number of protons, neutrons and electrons. 7. 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 8. Which of the following statements concerning isotopes is true? a. Every element has 3 isotopes.
2 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. 9. 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 10. Which element can be classified as a transition metal? a. N b. Ne c. Ce d. Co e. Sr 11. Which of the following compounds has the same percent composition by mass as styrene, C 8 H 8? a. acetylene, C 2 H 2 b. benzene, C 6 H 6 c. cyclobutadiene, C 4 H 4 d. α-ethyl naphthalene, C 12 H 12 e. all of these 12. The empirical formula of a group of compounds is CHCl. Lindane, a powerful insecticide, is a member of this group. The molar mass of lindane is g/mol. How many atoms of carbon does a molecule of lindane contain? a. 2 b. 3 c. 4 d. 6 e Which formula name combination is incorrect? a. CS 2 carbon disulfide b. Cl 2 O 7 chlorine heptaoxide c. P 2 O 3 diphosphorus trioxide d. SbCl 5 antimony pentachloride e. SCl 2 sulfur dichloride 14. 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 15. Give the formula for the ionic compound that forms between strontium and nitrogen. a. SrN d. SrN 2 b. Sr 2 N e. Sr 3 N 2 c. Sr 3 N 16. Which compound is incorrectly named? a. Ba(OH) 2 barium hydroxide b. Na 2 SO 4 sodium sulfite c. Fe(OH) 3 iron(iii) hydroxide d. NH 4 NO 3 ammonium nitrate e. K 2 Cr 2 O 7 potassium dichromate 17. In the reaction given below, if 10 moles of aluminum oxide are consumed, how many moles of oxygen gas are produced? 2 Al 2 O 3 4Al + 3O 2 a. 3 d. 12 b. 4 e. 15 c Classify the following reaction.
3 2 Al(s) + 3 CuSO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 3 Cu (s) a. combination b. decomposition c. displacement d. exchange 19. Classify the following reaction. CaCl 2 (aq) + H 2 SO 4 (aq) CaSO 4 (s) + 2 HCl (aq) a. combination b. decomposition c. displacement d. exchange 20. The Roman numerals in the reaction given represent the coefficients in the balanced chemical equation. What are the values of the coefficients? I II III IV a b c d e I CO + II NO III CO 2 + IV N Which of the following is a diprotic acid? a. H 3 PO 4 b. H 2 O c. Ca(OH) Which compound can be classified as insoluble: a. Mg(ClO 4 ) 2 ; b. Cu(NO 3 ) 2 ; c. (NH 4 ) 2 SO 4 ; d. H 2 SO 4 e. NH 3 d. Fe(OH) 2 e. NiSO 4 ; 23. Consider the reaction between 15.0 ml of a 1.00 M aqueous solution of AgNO 3 and 10.0 ml of a 1.00 M aqueous solution of K 2 CO 3. When these react, a precipitate is observed. What is present in solution after the reaction is complete (Note: the precipitate is not considered to be in solution)? a. Ag +, NO 3, K +, CO 2 3, water b. Ag +, NO 3, K +, water c. K +, CO 2 3, water 24. Which statement about strong acids is true? a. Strong acids are weak electrolytes. b. Strong acids are very concentrated. c. Strong acids are almost entirely converted to ions when dissolved in water. d. Acetic acid is a strong acid. e. All of the above are true. d. NO 3, K +, CO 2 3, water e. water 25. Which of the following is a weak electrolyte? a. HBr b. Mg(OH) 2 c. NaCl 26. What is the correct formula for the ammonium ion? a. H 2 O; b. H 3 O + ; c. OH - ; 27. What is the oxidation state (number) of Cl in Cl 2 O 5? a. +2 b. -2 d. HCN e. KI d. H - ; e. NH + 4 ; c. -3 d. -5
4 e What is the oxidation state (number) of P in PO 4 3-? a. -3 b. +3 c What is the oxidation state (number) of H in MgH 2? a. -1 b. +1 c What is the oxidation state (number) of F in F 2? a. -1 b. +1 c. 0 d. +5 e. 5 d. +2 e. 2 d. +2 e. 2 PART II: SHORT ANSWER (Each short answer question has a 1-point value!!) 31. Molarity is defined as the number of moles of solute per volume of solution in liters. 32. Ions that contain atoms of more than one element are called polyatomic ions. 33. Proton donors are known as Brønsted acids. 34. A compound that forms between a non-metal and a non-metal is a molecular compound. 35. Stoichiometric coefficients found in a balanced equation can be used to derive mole ratios. 36. When an element is oxidized, it loses electrons. 37. A neutralization reaction involves the reaction of a(n) acid with a(n) base. 38. A chemical equation without the spectator ions is called a net ionic equation. PART III: NOMENCLATURE OF MOLECULAR AND IONIC COMPOUNDS 39. (4 pts) Name the following compounds: a. Al 2 (SO 4 ) 3 Aluminum sulfate b. Cl 2 O 5 Dichlorine pentaoxide c. Li 2 HPO 4 Lithium hydrogen phosphate d. Ba(ClO 3 ) 2 Barium chlorate 40. (4 pts) Write the correct formula for each of the following compounds: a. Calcium Nitrite; Ca(NO 2 ) 2 b. Copper(I) Cyanide; CuCN c. Sodium Nitride; Na 3 N d. Potassium Permanganate; KMnO 4 PART IV: CHEMICAL REACTIONS 41. (9 pts) Write (clearly and distinctly) the complete and net ionic equations for each of the following reactions IN WATER. If no reaction occurs, write NR.
5 a. ZnCl 2 + Na 2 SO 4 NO Reaction, All potential products are soluble ionic compounds b. CuSO 4 (aq) + Na 2 CO 3 (aq) CuCO3 (s) + Na 2 CO 3 (aq) Cu 2+ (aq) + SO 4 2- (aq) + 2 Na + (aq) + CO 3 2- (aq) 2 Na + (aq) + SO 4 2- (aq) + CuCO 3 (s) Net: Cu 2+ (aq) + CO 3 2- (aq) (aq) CuCO 3 (s) c. CH 3 COOH (aq) + KOH (aq) CH 3 COOK (aq) + H 2 O (l) CH 3 COOH (aq) + K + (aq) + OH - (aq) CH 3 COO - (aq) + K + (aq) + H 2 O (l) Net: CH 3 COOH (aq) + OH - (aq) CH 3 COO - (aq) + H 2 O (l) 42. (4 pts) Find which of the equations below represent redox reactions. Indicate the element that is reduced and the element that is oxidized. Write their oxidation numbers before and after the reaction. a. NaNO 3 (s) + Pb (s) NaNO 2 (s) + PbO (s) redox reaction Oxidized: Pb; oxidation number changes from 0 to +2 Reduced: N; oxidation number changes from +5 to +4 b. Na 2 SO 4 (s) + 4 C (s) Na 2 S (s) + 4 CO (s) redox reaction Oxidized: C; oxidation number changes from 0 to +2 Reduced: S; oxidation number changes from +6 to -2 PART V: CONCEPTS 43. (4 pts) The following diagram represents the reaction of A 2 (gray spheres) with B 2 (white spheres): a. Write a balanced chemical equation for the reaction; A B 2 2 AB 3 b. Identify the limiting reactant; Limiting reactant: B 2 Reactant in excess: A 2 c. How many moles of product can be made from 1.0 mol of A 2 and 1.0 mol of B 2? 1 mol A 2 x 2 mol AB 3 /1 mol A 2 = 2 mol AB 3 1 mol B 2 x 2 mol AB 3 /3 mol B 2 = 2/3 mol AB 3 The maximum # of moles that can be prepared from 1mol A 2 and 1 mol B 2 (theoretical yield) is 2/3 mol AB 3 PART VI: CALCULATION PROBLEMS (Show your work in its entirety. Do not provide just a single number!). 44. (7 pts) Potassium superoxide, KO 2, is used in re-breathing gas masks to generate oxygen according to the equation: 4 KO 2 (s) + H 2 O (l) KOH (s) + O 2 (g) a. Balance the above equation; 4 KO 2 (s) + 2 H 2 O (l) 4 KOH (s) + 3 O 2 (g)
6 b. If a reaction vessel contains 0.25 mol of KO 2 and 0.15 mol of H 2 O, find the following: i. The limiting reactant in the process; Answer: Examine separately the amounts of a product generated from KO 2 and H 2 O individually. Since the next question is about the quantity of O 2, let us choose O 2 as the product to study: 1. Mol amount of O 2 from KO 2 : 0.25 mol KO 2 x (3 mol O 2 /4 mol KO 2 ) = 0.19 mol O 2 2. Mol amount of O 2 from H 2 O: 0.15 mol H 2 O x (3 mol O 2 /2 mol H 2 O) = 0.22 mol O 2 Therefore KO 2 is the limiting reactant. ii. The theoretical yield of O 2 (in grams!); Answer: In the previous part we found the theoretical yield of O 2 in moles. All we have to do is convert it to grams: 0.19 mol O 2 x (32.00 g O 2 /1 mol O 2 ) = 6.1 g O 2 iii. The excess amount of the reactant that is not the limiting one (in grams!); Answer: The other reactant, H 2 O, is in excess. Let us start with finding the amount of water consumed in the reaction: 0.25 mol KO 2 x (2 mol H 2 O/4 mol KO 2 ) x (18.02 g H 2 O/1 mol H 2 O) = 2.3 g H 2 O was consumed The total amount of H 2 O (in grams) is: 0.15 mol H 2 O x (18.02 g H 2 O/1 mol H 2 O) = 2.7 g Therefore the excess amount is: = 0.4 g of H 2 O c. Suppose the process occurred with a 90% yield of oxygen. What was the actual yield of O 2? Answer: It is based on the definition of percentage yield: 90% = (actual yield/theoretical yield) x 100 Therefore: Actual yield = (90 x 6.1)/100 = 5.5 g 45. (3 pts) BONUS PROBLEM (You do not have to solve this problem in order to get full credit. But in order to receive credit for this problem, it has to be solved entirely!!). Element X, a member of Group 5A, forms two chlorides XCl 3 and XCl 5. Reaction of an excess of Cl 2 with g of XCl 3 yields g of XCl 5. What is the atomic mass and the identity of element X? Solution: We are given the quantities of XCl 3 and XCl 5 and, in addition, we can write the balanced chemical equation: XCl 3 + Cl 2 XCl 5 According to that equation, the mole ratio is 1 mol XCl 5 /1 mol XCl 3. We also need to realize that Molar Mass of XCl 5 = (Molar Mass of XCl 3 + Molar Mass Cl 2 ) = (Molar Mass XCl ) = (Molecular weight XCl ) g/1 mol Let Molecular Weight XCl 3 = M Then Molar Mass XCl 3 = M g XCl 3 /1 mol and Molar mass XCl 5 = (M ) g XCl 5 /1 mol According to the balanced chemical equation: g XCl 3 x 1 mol XCl 3 /M g XCl 3 x 1 mol XCl 5 /1 mol XCl 3 x (M ) g XCl 5 /1 mol XCl 5 = g XCl 5 We have obtained an equation with one unknown. Solving for M we find: x (M )/M =
7 M = Thus, the molecular weight of XCl 3 is M = amu The combined weight of three Cl atoms is x 3 = The weight of element X = = Element X is Phosphorous (P).
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