chem1a-spring04-mt1-unknown-soln - Page 1 of 6 Name Part 1...

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Unformatted text preview: Page 1 of 6 Name:_________________________________________ Part 1: Multiple Choice. (4 pts each, 40 pts total) Instructions: Bubble in the correct answer on your Scantron™ form AND circle the answer on your exam. Each question has one correct answer. 1.) The answer to question 1 is A. Bubble in A on your Scantron™ form. 2.) Which ionic compound is comprised of isoelectronic ions? A.) NaBr 3.) C.) BeF2 D.) MgBr2 E.) CaCl2 Which compound has the highest percentage of chlorine by mass? A.) HCl 4.) B.) KI B.) KCl C.) MgCl2 D.) BaCl2 E.) AlCl3 What is the empirical formula of a hydrocarbon whose combustion products give the mass spectrum shown on the right? A.) C4H B.) C2H C.) CH D.) CH2 E.) CH4 18 44 Mass 5.) Which difluoropropane (C3H6F2) molecule is chiral? (note: the H atoms are not shown) F C C C A.) F 6.) A.) F F B.) C C C F F C C C C. ) F C C C D.) F E.) D.) E.) Which is the correct Lewis structure of hydrogen cyanide? B.) C.) Page 2 of 6 Which molecule does not have an electric dipole moment? A.) CO 8.) B.) NH3 C.) SiO2 D.) CH2F2 E.) ICl3 Viewed through a filter with the absorption spectrum shown, a yellow solution will appear: A.) Black B.) Blue C.) Green D.) Yellow Absorbance 7.) Name:_________________________________________ E.) Red Blue 9.) Green Red Two photons are absorbed and one is emitted as shown. What is the wavelength of the emitted light (nm)? 1200 nm A.) 200 C.) 600 B.) 400 D.) 800 E.) 1800 λ 600 nm Which emission spectrum corresponds to the energy level diagram shown? Energy 10.) A.) 11.) λ B.) C.) λ λ D.) λ The nucleus of which of the following exotic isotopes contains the most neutrons? A.) 38 Cl 17 B.) 40 18 Ar C.) 40 19 K D.) 40 20 Ca E.) 38 Sc 21 E.) λ λ Page 3 of 6 Name:_________________________________________ Part 2: Short Answer Problems (70 pts total) Instructions: Enter answers in the boxes provided. Show your work. Where requested, write explanations in fifteen words or less. (25 pts) Acetaldehyde molecules contain carbon : hydrogen : oxygen in the mass ratio 6 : 1 : 4. 1.) a) What is the mole percent of the elements in acetaldehyde? masses: C=12 O = 16 H=1 C:H:O mass: 6 : 1 : 4 24 : 4 : 16 moles: 2 : 4 : 1 C: H: O: Total: b) 29% 57% 14% 100% What is the empirical formula of acetaldehyde? C = 2/7 , H = 4/7, O = 1/7 Answer: C2H4O c) The molar mass of acetaldehyde is 44 g/mol. What is the molecular formula? Explain. Explanation: This molecular formula satisfies both the correct mole ratios and molar mass. Answer: C2H4O d) e) Draw the Lewis structure for acetaldehyde. (note: the molecule has a C−C single bond) Structure: H H | | H – C – C = O | H What is the approximate H−C−O bond angle in acetaldehyde? Explain. Explanation: C has stearic number 3. Answer: 120 Page 4 of 6 Name:_________________________________________ (30 pts) Consider the following reactions that you encountered in the laboratory: 2.) I) 1 C6H12O6 (s) + 6 O2 (g) II) 2 NaN3 (s) 6 H2O (l) + 6 CO2 (g) 2 Na (s) + 3 N2 (g) a) Balance the reactions by writing the coefficients in the spaces provided above. b) One mole of each of the reactants in reaction I is placed in a baggie. Which is the limiting reactant? Explain. Explanation: 6 moles of O2 are required to react with 1 mole C6H12O6 c) Answer: O2 What mass of water is produced by the reaction in part b)? Answer: 18 g d) Separate baggies are prepared for reactions I and II with 1 mole of each of the reactants. Plot the baggie volume versus time for each of the reactions. (note: you may neglect the volume of the solids and liquids and assume that a mole of gas occupies a volume of 22.4 L) e) Explain why reaction II is better suited than reaction I for automobile airbags. Explanation: Reaction 2 produces more gas per mole of reactant. Page 5 of 6 Name:_________________________________________ Electron Kinetic Energy (15 pts) 3.) Consider the following plot depicting the photoelectric effect for Mg metal: Mg υ 0 a) υo (7.25 x 1014 Hz) Will yellow light (600 nm) eject electrons from Mg? Explain. Explanation: ν = c /=λ = 5 x 1014 Hz (for λ = 600 nm) = 5 x 1014 Hz < 7.25 x 1014 Hz Answer: No b) Draw a line on the plot above for Cs metal which has a work function (Φ) equal to one half of the work function of Mg. c) If 1.00 x 1015 Hz light is used, electrons from which metal will have a longer de Broglie wavelength, Mg or Cs? Explanation: Mg will have lower p; lower p => greater λ Answer: Mg ...
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