First initial of last name Chem. 1A Midterm 1 Version A October 20, 2017 Name: Print Neatly. You will lose 1 point if I cannot read your name or perm number. Perm Number: All work must be shown on the exam for partial credit. Points will be taken off for incorrect or missing units. Calculators are allowed. Cell phones may not be used as calculators. On fundamental and challenge problems you must show your work in order to receive credit for the problem. If your cell phone goes off during the exam, you will have your exam removed from you. Fundamentals (of 36 possible) Problem 1 (of 17 possible) Problem 2 (of 17 possible) Multiple Choice (of 30 possible) Midterm Total (of 100 possible) 1
Fundamental Questions Each of these fundamental chemistry questions is worth 6 points. You must show work to get credit. Little to no partial credit will be awarded. Make sure to include the correct units on your answers. 1) 6 pts How many significant figures does 0.000123045560 have? 9 Do the follow calculation, making sure to report your answer to the correct significant figures. 12.0(11.90 11.8) = 1 1.203 10 6 = 3.58 109 0.000360 2.40 10 5 2a) 2 pts How many es in 5.23 10 23 atoms? 5.23 10 23 atoms ( 1 6.0224 10 23 atoms 2b) 2 pts How many es of copper in 3.20 g? 3.20 g Cu ( 1 Cu 63.55 g Cu ) = 0.0504 Cu 2c) 2 pts How many es of O in 3.4 of CuSO4? 4 O 3.4 CuSO 4 ( ) = 14 O 1 CuSO 4 ) = 0.868 3a) 3 pts Write the balanced chemical reaction, including states for the following chemical reaction: Aqueous lead (IV) nitrate reacts with aqueous sodium sulfate to yield a lead (IV) sulfate precipitate and soluble sodium nitrate. Pb(NO3)4(aq) + 2Na2SO4(aq) Pb(SO4)2(s) + 4NaNO3(aq) 3b) 3 pts Write the word equation for the following reaction. You must use the correct IUPAC names. N2O4(g) + H2O(l) HNO2(aq) + HNO3(aq) Gaseous dinitrogen tetroxide combines with liquid water (dihydrogen oxide) to yield nitric acid and nitrous acid. 2
4) 6 pts Gaseous ethane CH3CH3 reacts with gaseous oxygen gas O2 to produce gaseous carbon dioxide CO2 and gaseous water H2O. If 1.08 g of water is produced from the reaction of 1.2 g of ethane and 8.6 g of oxygen gas, calculate the percent yield of water. Be sure your answer has the correct number of significant digits. Balanced Equation: 2CH3CH3(g) + 5O2(g) 2CO2(g) + 6H2O(g) Calculate es of CH3CH3 and O2 1.2 g CH 3 CH 3 ( 1 CH 3CH 3 30.0694 g CH 3 CH 3 ) = 0.040 CH 3 CH 3 8.6 g O 2 ( 1 O 2 31.998 g O 2 ) = 0.27 CH 3 CH 3 Find limiting reagent 0.040 CH 3 CH 3 ( 5 O 2 ) = 0.10 O 2 CH 3 CH 2 3 CH3CH3 is the limiting Reagent CH3CH3 (L.R.) O2 CO2 H2O Initial () 0.040 0.27 0 0 Change -2x=-0-5x= +2x=+0.040 +6x=+0.12 () -0.10 Final () 0 0.117 0.040 0.12 0.040 2x = 0 x = 0.020 Determine percent yield 0.12 H 2 O ( 18.0148 g H 2O 1 H 2 O 2O 1.08 g Percent Yield = ( ) 100% = 49% 2.2 g 5) 6 pts What is the mass percentage of Cl in NaCl? mass % Cl = m Cl 100% m total Assume 1 e NaCl Calculate mtot Calculate mcl 1 NaCl ( 1 NaCl ( Calculate mass % Cl mass % Cl = 58.443 g NaCl 1 NaCl ) = 58.443 g NaCl 1 Cl g Cl ) (35.453 1 NaCl 1 Cl ) = 35.453 g Cl 35.453 g 100% = 60.663% Cl 58.443 g 6) 6 pts a) How many protons, electrons, and neutrons does 127 I have 53 protons, 54 electrons, and 74 neutrons b) Give an example of two species that are isotopes of each other. Carbon-12 and Carbon-14 3
Challenge Problems Each of the following short answer questions are worth the noted points. Partial credit will be given. You must show your work to get credit. Make sure to include proper units on your answer. 1) 17 pts Ammonia react with O2 to form either NO(g) or NO2(g) according to these unbalanced equations: NH3(g) + O2(g) NO(g) + H2O(g) NH3(g) + O2(g) NO2(g) + H2O(g) In a certain experiment, 2.00 es of NH3(g) and 10.00 es of O2(g) are contained in a closed flask. After the reaction is complete, 6.75 es of O2(g) remains. Calculate the number of es of NO(g) in the product mixture. (Hint: You cannot do this problem by adding the balanced equations because you cannot assume that the two reactions will occur with equal probability.) You need to determine the amount of NO(g) produced. You know that 2.00 of NH 3 are reacted with 10.00 of O 2 and 6.75 es of O 2 remains after the reaction goes to completion. This indicates that NH 3 is the limiting reagent Determine the es of O 2 reacted n O2 (reacted) = n O2 (original) n O2 (unreacted) = 10.0 6.75 = 3.25 NH 3 reacts by one of the following equations 4NH 3(g) + 5O 2(g) 4NO(g) + 6H 2O(g) 4NH 3(g) + 7O 2(g) 4NO 2(g) + 6H 2O(g) Therefore for equation 1 n O2 = 5 4 n NO and for equation 2 n O2 = 7 4 n NO 2 The total number of es of O 2 used was 3.25 giving 5 4 n NO + 7 4 n NO 2 = 3.25 Similarly for equation 1 n NH3 = n NO and for equation 2 n NH3 = n NO2 The total number of es of NH 3 used was 2.00 giving n NO + n NO2 = 2.00 Combine equations and solve for e of NO n NO2 = 2.00 n NO 5 4 n NO + 7 4 (2.00 n NO) = 3.25 5n NO + 14.0 7n NO = 13 2n NO = 1 n NO = 0.50 4
2) 17 pts A compound contains only carbon, hydrogen, and oxygen. Combustion of 10.68 mg of the compound yields 16.01 mg CO2 and 4.37 mg H2O. The ar mass of the compound is 176.1 g. What are the empirical and ecular formulas of the compound? CxHyOz + O2 CO2 + H2O (Combustion Reaction) All of the C in the compound goes into forming CO2. Therefore, n CO2 = n C. 16.01 mg CO 2 ( 1 g ) ( 1 CO 2 1 C ) ( ) 1000 mg 44.01 g CO 2 1 CO 2 = 3.638 10 4 C All of the H in the compound goes into forming H2O. Therefore, n H2 O = 2n H. 4.37 mg H 2 O ( 1 g ) ( 1 H 2O 2 H ) ( ) 1000 mg 18.02 g H 2 O 1 H 2 O = 4.85 10 4 H To find the mass of O, find the mass of C and H, and subtract them from the overall weight of the compound. Calculate m C m C = 3.638 10 4 12.01 g C C ( ) = 0.004369 g C 1 C Calculate m H m H = 4.85 10 4 1.01 g H H ( ) = 4.90 1 H 10 4 g H Calculate m O m O = m Cx H y O z m C m H m Cx H y O z = 10.68 mg ( 1 g 1000 mg ) = 0.01068 g m O = 0.01068 g 0.004369 g 4.90 10 4 g = 0.00582 g Calculate n O 1 O 0.00582 g ( ) = 3.64 16.00 g O 10 4 O Divide through by smallest e amount (3.64 10-4 ) Carbon Hydrogen Oxygen 3.638 10 4 = 0.999 4.85 10 4 = 1.33 3.64 10 4 = 1.00 3.64 10 4 3.64 10 4 3.64 10 4 Multiple through by 3 to get whole numbers Empirical Formula C3H4O3 M C3 H 4 O 8 = 88.07 g Find ecular formula g 176.1 g 88.07 C6H8O6 = 2.000 Multiply empirical formula by 2 5
Multiple Choice Questions On the ParScore form, you need to fill in your answers, perm number, test version, and name. Failure to do any of these things will result in the loss of 1 point. Your perm number is placed and bubbled in under the ID number. Do not skip boxes or put in a hyphen; unused boxes should be left blank. Bubble in your test version (A) under the test form. Note: Your ParScore form will not be returned to you, therefore, for your records, you may want to mark your answers on this sheet. Each multiple-choice question is worth 5 points. 1. For a new element, 67.16% is an isotope with mass 280.8 amu, 2.76% is an isotope with mass 283.7 amu, and 30.08% is an isotope with mass 284.8 amu. Calculate the average atomic mass of this new element. A) 313.4 amu B) 283.1 amu C) 282.1 amu D) 280.8 amu 2. An oxybromate compound, KBrOx, where x is a whole number, is analyzed and found to contain 47.84% Br by mass. What is x? A) 0 B) 1 C) 2 D) 3 3. Which of the following statements is true? A) Halogen atoms tend to gain one electron to form 1 + ions B) Group 16 atoms tend to lose two electrons to form 2 + ions. C) Alkali metals tend to lose one electron to form 1 - ions. D) More than one statement is true. E) None of the previous statements are true. 4. A reaction of 3L of X4 gas and 4L of Y gas yields 4L of product. There is no excess X4 or Y gas after the reaction goes to completion. All of the gas volumes are at the same temperature and pressure. What is the formula of the gaseous product? A) X3Y B) X4Y C) X2Y2 D) X5Y 6
5. Which of the following is not the correct chemical formula for the compound named? A) ZnS zinc sulfide B) Al(OH)2 aluminum hydroxide C) Mg(C2H3O2)2 magnesium acetate D) Fe2O3 iron(iii) oxide 6. According to the law of definite proportions, A) it is not possible for the same two elements to form more than one compound. B) the total mass after a chemical change is the same as before the change. C) the ratio of the masses of the elements in a compound is always the same. D) if the same two elements form two different compounds, they do so in the same ratio. C,D,E,A,B,C 7