# Chapter 1 The Atomic Nature of Matter: Selected Answersc for Practice Exam.

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1 Chapter 1 The Atomic Nature of Matter: Selected Answersc for Practice Exam. MULTIPLE CHOICE g of dioxane (C 4 H 8 O 2 ) is how many moles of dioxane? mol mol mol d mol Strategy: add up the atomic weights of dioxane to obtain the molecular weight of dioxane. Then divide 5.80 g of dioxane into the molecular weights of dioxane to obtain the number of moles of dioxane. 4C + 8H + 2H = 88 gmol g/88 gmol -1 = mol. 51. How many molecules of tetrahydrofuran (C 4 H 8 O) are present in 2.00 x 10-9 mol of tetrahydrofuran? 1.67 x x x d x Strategy: There are 6.02 x molecules in a mol of tetrahydrofuran. Multiply this number by the number of moles of tetrahydrofuran x 10-9 mol x 6.02 x molecules mol -1 = 1.20 x molecules x10-3 mol of an unknown compound has a mass of g. The compound could be CH 4 C 2 H 6 C 3 H 8 d. C 4 H 10 Strategy: the number of g of the unknown divided by the MW of the compound equals the number of moles of the unknown, so the MW is the number of g divided by the number of moles. MW = g/8.55 x 10-3 mol = 16 g mol -1. CH 4 has a molecular weight of 16. Selected answers. P. 1

2 66. What is the atomic percentage of sulfur in a compound with the empirical formula SF 6? 21.95% 25.23% 29.67% d % Strategy: compute the MW of SF 6 (146) and then divide the AW of S (32) by the MW. %S = 32.0/146 x 100% = 21.9 % IMPORTANT. THE WRONG ANSWER WAS GIVEN IN THE PREVIOUS VERSION OF THE ANSWERS. 67. Compute the percentage by mass of carbon in a compound with the empirical formula C 2 H 2 O % 40.00% 57.14% d % ANS:C Strategy: compute the relative MW of C 2 H 2 O and then divide the total AW in C 2 H 2 O by the MW. 24/42 x 100% = % 71. A g sample of a compound is analyzed and found to contain g of phosphorus and g of oxygen. The empirical formula for this compound is PO 3 P 2 O P 2 O 5 d. P 3 O 2 Strategy: compute the number of moles of P and O in the compound. The ratio of the number of moles is equal to the empirical formula when the number of atoms are in the ratio of whole numbers. P = g/ gmol -1 = mol O = g/16.00 gmol -1 = 1.76 mol The ratio of P to O is 1 to 2.5 or 2 to 5 in small whole numbers. Therefore the empirical Formula is P 2 O 5 (c above). Selected answers. P. 2

3 76. The empirical formula for a hydrocarbon is found to be CH 2. In a separate experiment, the molar mass is determined to be approximately 112 g mol -1. The number of hydrogens in the molecular formula of the hydrocarbon is therefore d. 20 The relative MW of CH 2 is 14 g mol -1. The actual MW is 112 g mol -1. Divide the empirical MW into the actual MW to obtain the number of CH 2 units. 112/14 = 8 so the actual Molecular formula is (CH 2 ) 8 = C 8 H 16. Thus, there are 16 H atoms in the molecule. 77. Under certain conditions of temperature and pressure, 1.00 mole of any gas occupies a volume of 22.4 L. Thus, under these same conditions, mol of any gas will occupy a volume of 1.00 L 10.0 L 22.4 L d L Strategy: the number of moles for gases at the same pressure and temperature is directly Proportional to the volume. Thus, mol of a gas will equal the number of moles Multiplied by 22.4 L mol x 22.4 L mol -1 = 1.00 L 12. How many molecules are present in 36.5 g of CO 2 (molar mass = g/mol)? (a) 8.29 x 10-1 molecules (c) 5.00 x molecules (b) 5.00 molecules (d) 7.84 x molecules (e) None of the above Strategy: a mole of any substance contains 6.02 x molecules g of CO 2 = mol of CO 2. this is close to a mol so by inspection the only possible answer is 5.00 x molecules, i.e., mol x 6.02 x molecules mol Nitrogen exists as molecules of formula N 2. All of the following statements are true but one. Which is not true? (a) 28 g of nitrogen contains 6.02 x molecules. (b) 14 g of nitrogen is a mole of nitrogen atoms. (c) One mole of nitrogen molecules weighs 14 g. (d) 6.02 X nitrogen atoms weigh 14 G. (e) None of the above is incorrect. Since nitrogen exists as N 2 molecules, one mole of N 2 has a molecular weight of 2 x 14 = 28 g mol -1. Selected answers. P. 3

4 Chapter 2 Stoichiometry. Selected Answers MULTIPLE CHOICE 4. Consider the unbalanced chemical equation for the complete combustion of butanoic acid (C 4 H 8 O 2 ): C 4 H 8 O 2 + O 2 -> CO 2 + H 2 O. When the reaction is balanced using smallest integer coefficients, the coefficient for O 2 is d. 6 Strategy: Balance the atoms which appear the least number of time or appear in the most Complex formula first. Thus, balance C and H first. This leads to a balance of the C and H atoms. Then balance the O atoms. This leads to the final equation: C 4 H 8 O O 2 -> 4 CO H 2 O. Coefficient of O is Ammonia (NH 3 ) reacts with oxygen to produce nitrogen oxide (NO) and water. If this reaction is described with a balanced chemical equation using smallest integer coefficients, the coefficient for nitrogen oxide is d. 4 ANS: D Use the same strategy as in 4 and the balance equation becomes 4 NH 3 + 5O 2 -> 4 NO + 6 H 2 O 6. Propene gas (C 3 H 6 ) burns completely in oxygen to produce carbon dioxide and water as the only products. If this reaction is described with a balanced chemical equation using smallest integer coefficients, the coefficient for water is d. 9 Use the same strategy as the above two questions and the balanced equation becomes Selected answers. P. 4

5 2 C 3 H O 2 -> 6 CO H 2 O 10. The complete combustion of gaseous ethane (C 2 H 6 ) in oxygen yields just carbon dioxide and water. When this reaction is balanced using smallest integer coefficients, and these coefficients are interpreted as moles, what is the total mass of the reactants? 62 g 126 g 142 g d. 284 g ANS:D Strategy: Balance the equation and then compute the weight of the reactants from the Coefficients. Balanced equation 2 C 2 H O 2 -> 4 CO H 2 O 2 C 2 H 6 (2 x 30 g) + 7 O 2 (7 x 32 g) = 60 g g = 284 g. 21. Consider the production of ammonia gas through the reaction of nitrogen with hydrogen as described by the balanced equation, N 2 (g) + 3 H 2 (g) = 2 NH 3 (g) How much N 2 would be required to react completely with 1.50 mol of H 2? 14.0 g 28.0 g 42.0 g d. 126 g Strategy: from the balance equation, 3 mol of H 2 reacts with 1 mol of N 2, so 1.50 mol of H 2 reacts with 0.50 mol of N 2. The weight of 0.50 mol of N 2 is 14 g. Selected answers. P. 5

6 23. Consider the production of ammonia gas through the reaction of nitrogen with hydrogen as described by the balanced equation, N 2 (g) + 3 H 2 (g) = 2 NH 3 (g) If all gas volumes are measured under the same conditions of pressure and temperature, what volume of ammonia gas would be produced by the complete reaction of 40.0 L of nitrogen? 20.0 L 26.7 L 40.0 L d L ANS: D Strategy: volumes of gases reacting are directly proportional to the number of moles of Gases reacting. Since 1 mol of N 2 produces 2 moles of NH 3, 40 L of N 2 produces 80 L of NH The gaseous elements H 2 and O 2 react explosively to form water (H 2 O) L of H 2 is mixed with 2.00 L of O 2 and the mixture is ignited in a strong steel vessel. If the gas volumes are all measured at the same temperature and pressure, which gas remains unreacted, and what is its volume? O 2, 0.50 L O 2, 1.00 L H 2, 1.00 L d. H 2, 2.00 L ANS:A Strategy: from the equation 2 H 2 + O 2 -> 2 H 2 Ol, 2 L of H 2 will react with 1 L of O 2, so 3 L of O 2 will react completely with 1.5 L of O 2 leaving 0.5 L of O 2 unreacted. Selected answers. P. 6

7 Chapter 3 Lewis Structures. Selected Answers 15. In which of the following atoms is the number of valence electrons equal to six? P Se Sb ANS: B Se (atomic number, AN = 34 is in group 6 and therefore has 6 valence electrons in the atom. P (AN = 15) and Sb (AN = 51) are in group 5 and have 5 valence electrons in the atom. 16. In which of the following atoms is the number of valence electrons equal to three? Ge Sn Pb ANS: E Ge (AN = 32), Sn (AN = 50) and Pb (AN = 82) are all in group 4 and have 4 valence electrons. 19. Which of the following is the correct Lewis dot symbol for the gallium atom? d. Ga (AN = 31) is in group 3 and therefore has 3 valence electrons. Selected answers. P. 7

8 20. Which of the following is the correct Lewis dot symbol for the selenium atom? d. Se (AN = 34) is in group 6 and has 6 valence electrons. 23. Which of the following is the correct Lewis dot symbol for the P 2+ ion? d. ANS: E Strategy: consider the number of valence electrons in the atom (equal to the group number) and then note that Addition of an electron will produce a negatively charged atom and removal of an electron will produce a positive Charge. Thus, P 2+ has two less valence electrons than the atom. Since P is in group 5, P 2+ has 3 valence electrons. None of the answers have 3 valence electrons. Selected answers. P. 8

9 24. Which of the following is the correct Lewis dot symbol for the S - ion? d. ANS: B Following the same strategy as 23, S - will have one more electron than the atom S. Since S is in group 6 the atom has 6 valence electrons and S - will have 7 valence electrons. 31. Which of the following Lewis structures contains an error? ANS: B IMPORTANT: THERE IS A TYPO IN THIS QUESTION. BOTH A AND B CONTAIN ERRORS. HCN MUST HAVE 10 VALENCE ELECTRONS BUT THE STRUCTURE SHOWN FOR A ONLY HAS 8. THE STRUCTURE SHOWN FOR B HAS 12 VALENCE ELECTRONS BUT THE NEUTRAL COMPOSITION C 2 H 2 HAS ONLY 10 VALENCE ELECTRONS. MY GUESS IS THAT A SHOULD HAVE A TRIPLE BOND BETWEEN C AND N AND THAT THE FORMULA SHOWN IS A TYPO. Selected answers. P. 9

10 32. Which of the following Lewis structures contains an error? ANS: E CO 2 should have 16 valence electrons and all octets. CO should have 10 valence electrons and all octets. F 2 should have 14 valence electrons and all octets. All of the structures fulfill these requirements. 33. What is the formal charge on the nitrogen atom in the Lewis structure,? d. +2 From the formula for formal charge, FC = group number [number of unshared electrons + 1/2 number of shared Electrons. FC (N) = 5 4 = +1, FC (C) = 4 5 = -1, FC (O) = 6 7 = What is the formal charge on the bromine atom in the Lewis structure, d. +5 ANS: B From the formula for formal charge, FC (Br) = 7 5 = +2. Selected answers. P. 10

11 35. What is the formal charge on the nitrogen atom in the Lewis structure,? d. +2 ANS: B FC (N) = 4 4 = 0 Selected answers. P. 11

12 36. Which of the following is a correct Lewis dot structure for OF 2? d. ANS: D The neutral formula OF 2 has x 7 = 20 valence electrons. Only D has 20 valence electrons. 37. Which of the following is a correct Lewis dot structure for O 2 2-? IMPORTANT. THIS IS ANOTHER TYPO. THE CORRECT ANSWER IS E. O 2 2- SHOULD HAVE 14 VALENCE ELECTRONS. A HAS 18! 39. Which of the following Lewis structures can be drawn as two or more resonance forms? ANS: D RATS, STILL ANOTHER TYPO. A CAN BE WRITTEN IN A SECOND RESONANCE Selected answers. P. 12

13 FORM AND SO CAN C. ALTHOUGH B CAN BE WRITTEN IN A SECOND RESONANCE FORM IT DOES NOT HAVE THE CORRRECT NUMBER OF VALENCE ELECTRONS FOR N 3 -. CAN YOU FIGURE OUT HOW MANY VALENCE ELECTRONS B HAS AND HOW MANY N 3 - HAS? 40. Which of the following must have a Lewis dot structure that violates the octet rule? NO 2 SO 2 CO 2 NO 2 has 17 valence electrons and therefore is an odd electron molecule. It cannot fulfill the octet rule about All of its atoms. SO 2 and CO 2 can fulfill the octet rule about all the atoms. Selected answers. P. 13

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