Houston Community College System

. Houston Community College System Chemistry 1311 EXAM # 1A The Periodic table of the Chemical Elements 1

CHEM 1311 Sample Exam #1A (1,2, and 3) Part I. Multiple choice questions. ( 4 points each) Direction- Please write your correct choice in space provide. 1. The distance between carbon atoms in ethylene is 134 picometers. Which of the following express the distance in meters? A. 1.34x10-13 m B. 1.34x10-10 m C. 1.34x10-7 m D. 1.34x10-6 m 2. Acetic acid boils at 244.2 0 F. What is boiling point in degrees celsius? A. 167.7 0 C B. 153.4 0 C C. 117.8 0 C D. 103.7 0 C 3. Which of the following expressed measurement 52,030.2 m in correct scientific notation? A. 5.20302x10 4 m B. 5.203x10 4 m C. 5.20x10 4 m D. 5.2x10 4 m 4. Select the answer with correct number of decimal places for the following sum. 4.65 g + 2.0 g + 19.6442 g = A. 26.2942 g B. 26.29 g C. 26 g D. 26.3 g 5. Which of the following is a non-metal? A. Li B. Be C. Os D. I 6. Bromine is the only non-metal that is liquid at room temperature. Consider the isotope bromine-81, 35 81 Br. Select the combination which list the correct atomic number, neutron number, and mass number respectively. A. 35,46,81 B. 35,81,46 C. 81,46,35 D. 46,81,35 7. Which of the following name is correct for given chemical formula? A. I 2 O 5, iodine pentoxide B. LiNO 3, lithium nitrate C. PbO, lead(i) oxide D. H 2 SO4, hydrosulfuric acid 8. Which of the following chemical formula is not correct for given chemical name? A. ferric oxide, Fe 2 O 3 B. calcium hydroxide, Ca(OH) 2 C. tetrasulfur dinitride, S 4 N 2 D. potassium chlorite, KClO 3 9. What is the mass in grams of 0.250 mole common antiacid, calcium carbonate? A. 4.00x10 2 g B. 25.0 g C. 4.00x10-2 g D. 2.50x10-3 g 2

10. Which one of the following reactions is not balanced? A. 2C 6 H 6(l) + 15O 2(g) 12CO 2(g) + 6H 2 O (g) B. B 2 O 3(s) + 6HF (l) 2BF 3(g) + 3H 2 O (l) C. UO 2(g) + 4HF (l) UF 4(s) + 4H 2 O (l) D. 2B 5 H 9(l) + 12O 2(g) 5B 2 O 3(s) + 9 H 2 O (g) 11. Which identification is INCORRECT for the element? A. Na, sodium B. S, sulfur C. P, potassium D. Zn, zinc 12. Which of the following is not a chemical change? A. burning coal B. making bread rise using baking soda C. boiling an egg D. boiling water 13. Which ion has a -1 charge? A. CO 3 B. HSO 3 C. NH 4 D. SO 4 14. Which of the following is improperly labeled? A. Cu- transition metal B. Mg- alkaline earth metal C. Br- halogen D. U- noble gas 15. How many carbon atoms are in CH 3 -CH 2 -COO-CH 3 A. 1 B. 2 C. 3 D. 4 Part II. Show all your work for complete credit. 16. The density of mercury, the only metal to exist as liquid at room temperature, is 13.6 g/cm 3. What is the density in pounds per cubic inch (lb/in 3 )? ( 1 lb=454 g, 1 in =2.54 cm) ( 8 points) 17. The speed needed to scape the pull of earth, s gravity is 11.3 km/s. What is this speed in mi/hr? ( 1 mi=1.6094 km) ( 6 points) 18. Lithium, is used in dry cells and storage batteries and in high temperature lubricants, it has two naturally occurring isotopes, 6 Li and 7 Li. Calculate the atomic mass of lithium. ( 8 points) isotopes isotopic mass(amu) abundance(%) 6 Li 6.01521 7.50 7 Li 7.016003 92.50 19. Terphthalic acid, used in the production of polyester fibers and films, is composed of carbon, hydrogen, and oxygen. when 0.6943 g of acid is subjected to combustion analysis 3

it produced 1.471 g CO 2 and 0.391 g H 2 O. What is its empirical and molecular formula if its molar mass is 172 g/mol. ( 8 points) 20. Aluminum reacts with oxygen to produce aluminum oxide which can be used as an absorbent, desiccant or catalyst for organic reactions. ( 10 points) 4 Al (s) + 3 O 2(g) 2 Al 2 O 3(s) A mixture of 82.4 g of aluminum (M=26.98 g/mol) and 117.65 g of oxygen (M=32.00 g/mol) is allowed to react. a) Identify the limiting reactant. b) Calculate the mass of aluminum oxide formed. c) Determine the mass of the excess reactant present in the vessel when the reaction is complete. d) What is the percentage yield for the reaction if reaction mixture produce 120 g of aluminum oxide? Bonus question( 10 points)- show all your work. a) When 11.2 g CaCO 3 reacts with excess hydrochloric acid, as below, 4.61 g of CO 2 is produced. What is the percent yield of CO 2? Do not type the units with your answer. CaCO 3 (s) + 2 HCl(aq) CO 2 (g) + CaCl 2 (aq) + H 2 O(l) (not balanced) b) Calcium reacts with oxygen to make calcium oxide, CaO. What is the theoretical yield of CaO(s) that could be prepared from 7.63 g of Ca(s) and 2.56 g of O 2 (g)? 4

CHEM 1311 SAMPLE EXAM # 1A - KEY (1, 2, and 3) PART-I 1. B (pico = 10-12, 134 x 10-12 m = 1.34 x 10-10 m) 2. C C = 5/ 9(F-32) = 5/ 9(244.2-32) = 5/ 9(212.2) = 117.9 o C 3. A 4. D sum = 26.2942 g, round off to the least number of decimal point (one), sum = 29.3 g 5. D 6. A 7. B LiNO 3 is Lithium nitrate 8. D potassium chlorate, KClO 3 9. B CaCO 3 = 1(40) + 1(12) + 3(16) = 100 g/ml 10. C 2H 2 O (l) 11. C P is the symbol for phosphorous 12. D boiling water is a phase change from liquid to vapor 13. B HSO 3 (bisulfite) 14. D Uranium is an inner transition metal 15. D four carbon atoms PART-II 16. 13.6 g 1 lb (2.54 cm) 3 ----------- x ------------ x ------------------ = 0.491 lb/ in 3 cm 3 454 g 1 in 3 17. 11.3 km 1 mi 3600 sec ------------- x ---------------- x ---------------- = 2.53 x 10 4 mi/hr sec 1.6094 km 1 hr 18. Ave. atomic wt. of Li = (7.50/100) (6.01521) + (92.50/100) (7.016003) = 6.95 a.m.u. 19. m c = (3/11) mco 2 = (3/11)(1.471) = 0.4012 g, m H = (1/9) mh 2 O = (1/9) (0.391) = 0.0434 g m O = m - (m c + m H ) = 0.6943 - (0.4012 + 0.0434) = 0.2497 g C = 0.4012/12 = 0.0334, 0.0334/0.01561 = 2.14 = 2 H = 0.0434/1 = 0.0434, 0.0434/0.01561 = 2.78 = 3 O = 0.2497/16 = 0.01561, 0.01561/0.01561 = 1 = 1 5

empirical formula is C 2 H 3 O = 2(12) + 3(1) + 1(16) = 43 X = ( 172/ 43) = 4, (C 2 H 3 O) 4 or C 8 H 12 O 4 20a) (4x27) g (3x32) g (2x102) g 4 Al + 3 O 2 2Al 2 O 3 82.4 g 117.65 g X g (82.4) (2) (102) Al 2 O 3 from Al = ------------------------ = 155.64 g = 156 g 4 x 27 (117.65) (2) (102) Al 2 O 3 from O 2 = ------------------------ = 250.00 g 3 x 32 So, Al produce smaller amount of Al 2 O 3. Al is the limiting reagent. b) 156 g Al 2 O 3 formed c) (4x27) g (3x32) g 4 Al + 3 O 2 2Al 2 O 3 82.4 g X g (82.4) (3) (32) O 2 used up in the reaction = ------------------------ 4 x 27 O 2 (excess) = 117.65-73.2 g = 44.4 g = 73.2 g 120 d) % yield = ----------- x 100 = 77% 156 BONUS QUESTION: a) CaCO 3 (s) + 2 HCl(aq) CO 2 (g) + CaCl 2 (aq) + H 2 O(l) (11.2 g, CaCO 3 ) x ( 1 mole CaCO 3 / 100 g Mwt. ) ( 1mole CO 2 /1 mole CaCO 3 ) (44 g CO 2 / 1 mole CO 2 ) = 4.928 g CO 2 % yield = ( g, actual value / g, theoretical value) x 100 = 93.5 % b) 2 Ca + O 2 2 CaO (7.63 g Ca) (1 mol Ca / 40 g) ( 1 molo 2 /2 mol Ca) ( 32 g O 2 / 1 mol O 2 ) = 3.052 g O 2 6

3.052 g O 2 requried to react woth 7.63 g Ca. Since have only 2.56 g O 2 in the reaction, therefore O 2 is limiting reagent. (2.56 g, O 2 ) ( 1 mol / 32 g O 2 ) ( 2 mol CaO / 1 mol O 2 ) (56 g CaO / 1 mol CaO ) = 8.96 g CaO, theoretical value of CaO Houston Community College System Chemistry 1311 EXAM # 1B Comparison of Temperature Scales 7

CHEM 1311 Exam #1B (1,2, and 3) Directions- please answer the following multiple-choice questions next to each number. Please show all your work for bonus question and part (II) questions in the space provided. Part (I) - Multiple Choice - (3 points each) 1. Which number definitively distinguishes one element from another element? A. charge number B. number of neutrons C. number of protons D. number of electrons 2. A 239.5g sample of Benzene is found to occupy a volume of 274ml. What is its density? A. 0.874 B. 1.99 C. 0.788 D. 1.58 3. Convert 3.50 ft to cm. A. 107 cm B. 2.9 x 10 2 cm C. 381.0 cm D. 503 cm 4. How many electrons, protons, and neutrons are in an atom of 197 Au 3+? A. 77 electrons, 79 protons, 117 neutrons B. 80 electrons, 78 protons, 116 neutrons C. 76 electrons, 79 protons, 118 neutrons D. 80 electrons, 80 protons, 115 neutrons 5. Which name is incorrect? A. HBr hydrobromic acid B. AlCl 3, aluminum (III) chloride C. MgS, magnesium sulfide D. Mg 3 N 2, magnesium nitrite 6. Which one of the following is improperly labeled? A. He (noble gas) B. K (alkali metal) C. Ca (transition metal) D. Br (halogen) 7. Calculate the formula weight of aniline, C 6 H 7 N. A. 93.g B. 85g C. 62g D. 49g 8. Forty centimeters is equal to how many nanometers(nm)? 8

A. 4.00 x 10 8 nm B. 4.00 x 10-10 nm C. 4.00 x 10 12 nm D. 4.00 x 10-15 nm 9. The state of matter with the highest measure of entropy or disorder is called state. A. solid B. gas C. liquid D. mixed 10. Octane burns according to the following equation: 2C 8 H 18 + 25O 2 16CO 2 + 18H 2 O How many grams of CO 2 are produced when 5.00 g of C 8 H 18 are burned? A. 0.351 g B. 1.93 g C. 15.4 g D. 40.0 g 11. The elements with the same atomic number but different atomic masses are called. A. reactants B. isotopes C. anions D. molecules 12. Which of these compounds is not ionic? A. Li 2 SO 4 B. NaF C. SO 3 D. all of these 13. What is the molar mass of cryolite (Na 3 AlF 6 )? A. 205.0 B. 210.0 C. 185.3 D. 104.2 14. How many Magnesium atoms are in 3.00 moles of Mg 3 (PO 4 ) 2? A. 5.42 x 10 24 B. 1.48 x 10 22 C. 2.99 x 10 21 D. 2.40 x 10 22 15. An oxide of iron has the formula Fe 3 O 4. What mass percent of oxygen does it contain? A. 0.72% B. 60% C. 27.6% D. 72.6% 16. Determine the empirical formula of a compound that has (by mass) 85.7% carbon, 14.28% hydrogen. A. CH B. CH 2 C. C 2 H D. C 3 H 6 17. The empirical formula of styrene is CH, its molecular weight is 104.1. What is the molecular formula of styrene? A.C 8 H 8 B. C 4 H 4 C. C 10 H 10 D. none of these 18. Give (in order) the correct coefficients to balance the following reaction: C 2 H 4 + O 2 CO 2 + H 2 O A. 1,2,2,4 B. 1,1,1,2 C. 2,2,2,2 D. 1,3,2,2 9

19. The melting point of Sulfur is 115 0 C. What is this on the Fahrenheit scale? A. 116 0 F B. 621 0 F C. 2032 0 F D. 239 0 F 20. When 125.0 g of ethylene ( C 2 H 4 ) burns in oxygen to give carbon dioxide and water, how many grams of CO 2 are produced? A. 250.0 g B. 393.0 g C. 425.6 g D. 57.50 g Part (II) - Show all your work. (8 points each) 21. A hypothetical pure metallic cube has a side of 3.00 cm and a mass of 9.0 grams. What is the density of this cube in ounces per cubic inch (oz/in 3)? (Remember, 16oz equal 1 pound, 2.2lbs = 1kg) 22. A substance, N x O y, has the composition by mass 69.57% O. Calculate the molecular formula of this substance if its molecular mass is 92 grams. 23. A reaction that produces crude iron from iron ore is shown below: Fe 3 O 4 (s) + 4CO (g) 3 Fe (s) + 4CO 2 (g) in How many moles of iron could be produced from the reaction of 2.5 mol Fe 3 O 4 and 3.0 mol of CO? Identify limiting reactant and determine how many moles of unreacted substance remained the reaction. 24. A 0.9301 g sample of a compound known to contain C,H, and O was burned in oxygen to yield 1.729 g of CO 2 and 0.3535 g of H 2 O. What is the molecular formula of the compound if the molar mass of compound is approximately 140 g. 25. Perform the following calculations and round off the answers to the proper significant figures. a) 14.86 ml + 15.0 ml + 14.980 ml = b) (42.927 g/ml)(9.00 ml) = 10

BONUS question-show all your work.(10 points) Naturally occurring element Y exists in two isotopic forms: Y-57 ( 57.977 amu ), Y-58 ( 58.977amu) Calculate the percentage of each isotope if the average atomic weight of X is 58.776 amu. CHEM 1311 SAMPLE EXAM # 1B - KEY (1, 2, and 3) Part (I) 1. C 6. C 11. B 16. B 2. A 7. A 12. C 17. A 3. A 8. A 13. B 18. D 4. C 9. B 14. A 19. D 5. D 10. C 15. C 20. B Part (II) 21. D = m/v = (9.0 g/27.00 cm 3 ) = 0.33 g/cm 3 (0.33 g/cm 3 ) ( 10-3 kg/ 1g) ( 2.2lb/ 1kg) ( 16 oz / 1lb) ((2.54cm) 3 /1in 3 )= 0.19oz/ in 3 22. N = 30.43% O = 69.57% moles N = (92g/14g)(0.3043) = 1.99 2 moles O = (92g /16g)(0.6957) = 4.00, So the molecular formula is = N 2 O 4 OR: N = 30.43/14 = 2.174 2/174/2.174 = 1 O =69.57/16 = 4.348 4.348/2.174 = 2; (empirical formula is NO 2 ) (NO 2 ) X (14 + 32)X = (46)X = 92; X = 2; (NO 2 ) 2 N 2 O 4 23. 1.0 mole 4.0 mole 3 mole 4 mole Stoichiometric Relations Fe 3 O 4 (s) + 4CO (g) 3 Fe (s) + 4CO 2 (g) 2.5 mole 3.0 mole X mole Stoichiometric molar ratio of reactants Fe 3 O 4 /4CO =1/4 = 0.25 11

Given 2.0 mole Fe 3 O 4 to 2.5 mole CO; molar ratio is 2.5/3.0 = 0.833 Fe 3 O 4 is in excess and CO is limiting reagent. So use CO for stoichiometric relations to other reactant and products. X (from Fe 3 O 4 ) X (from CO) (2.5)(3.0) = ---------------------- = 7.5 moles Fe 1.0 (3.0)(3.0) = ---------------------- = 2.25 moles Fe (CO is limiting reagent) 4.0 mol CO used in the reaction = ( 3.0 mol CO)( 1 mol Fe 3 O 4 mol/ 4 mol CO) = 0.75 mol 2. 5 0.75 = 1.75 mol Fe remain in the reaction 24. mc = 3/11(1.729) = 0.4715 g, C mh = 1/9 (0.3535) = 0.03927 g, H mo = 0.9301 ( 0.4715 + 0.03927) = 0.4194 g, O C (0.4715/0.9301)(140/12) = 5.9 = 6.0 H (0.03927/0.9301)(140/1) = 5.9 = 6.0 O (0.4194/0.9301)(140/16) = 3.9 = 4.0, Therefore the molecular formula is C 6 H 6 O 4 OR C = ( 0.4715/12) = 0.03929 ( 0.03929/0.02621) = 1.49 =1.5; 1.5 x 2 = 3 H = (0.03927/1) = 0.03927 (0.03927 /0.02621) = 1.49 = 1.5; 1.5 x 2 = 3 O = (0.4194/ 16)= 0.02621 (0.02621/0.02621) = 1.00 = 1.0; 1 x 2 = 2 The empirical formula is ; C 3 H 3 O 2 (C 3 H 3 O 2 ) n = 140, (71) n = 140, n = 140 /71, n = 2 the molecular formula is ; C 6 H 6 O 4 25. a) 15.0 ml is the number with the least number of decimal places. Answer is 44.8 ml b) 9.00 ml is the number with the least number of significant figures(3 sig. Fig.) Answer is 386 g BONUS 12

Let fractional abundance of isotope X-57 be A; then 58.776 = 57.977 (Y/100) + 58.977(100 Y/100) Y = 20.1% for isotope X-57. and % abundance of isotope X-58 is 79.9%. 13