FinalKEY_1A_Spr2022

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NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 1

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 2 Potentially useful info: ? = ? ? 1 ? 2 ? 2 ?? = 1 2 ?? 2 = 3 2 ?? ?𝑉 = ??? ? ????? = ? ? + ? ? = 𝑋 ? ? ????? + 𝑋 ? ? ????? ? = ? ? ∆? ∆? ? = ∑ ?∆? ? ? (????????) − ∑ ?∆? ? ? (?????????) ∆? ? = ∑ ?? ? (????????) − ∑ ?? ? (?????????) = ∆? ? ? ∆? ? = ∑ ?∆? ? ? (????????) − ∑ ?∆? ? ? (?????????) ∆? ? = ∆? ? − ?∆? ? = − ? ? ∆? ? = − ? ? ln ? ? = ? ???𝑤??? ? ????𝑤??? = [????????] ? [?????????] ? ?? = − log[? + ] [? + ] = 10 −?𝐻 ? 𝑤 = [? + ][?? − ] = ? ? × ? ? = 1 × 10 −14 ?? + ??? = ?? ? + ?? ? = 14 ? = ??? ?

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 3 ? = 𝜈 × 𝜆 ? = ℎ 𝜈 ? ?𝑖?ℎ? − ? ?ℎ???ℎ??? = ?? ???????? ???? ????? = 1 2 ⁄ (# ????𝑖?𝑔 ? − − # ???𝑖????𝑖?𝑔 ? − ) ? 0 = 6.02214 × 10 23 ??? –1 ? = 96.5 ?? 𝑉 ??? ? – ℎ = 6.626 × 10 −34 ? ? ?(?) = ?( ? 𝐶) + 273.15 1 𝑉 = 1 ? ??? −1 1000 ?? = 1 ? 1 ??? = 101325 ?? = 760 ???𝑔 = 760 ???? ≈ 1 ??? ? = 8314.5 ?? ? ? –1 ??? –1 = 0.0821 ? ??? ? –1 ??? –1 ???: 1 ??? ??? 273.15 ?

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NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 4 On Exam 3, you encountered a skin cream containing a zinc oxide (ZnO) suncreen. Another common skincare ingredient with sun blocking properties is titanium dioxide (TiO 2 ). 1. First consider the titanium ion. What is its charge in this context? ☐ 4+ ☐ 2+ ☐ 0 ☐ 2 – ☐ 4 – 2. How many total electrons (core + valence) does the titanium ion have? ☐ 14 ☐ 18 ☐ 22 ☐ 26 ☐ 28 3. Arrange the titanium ion ’s electrons on the axis below. The energies of each subshell have been provided (not to scale), but you will need to label each subshell (1s, 2s, etc.) and populate it with the approriate number of electrons. You may have more energy levels than you need. 4. Sketch the approximate shape of the highest energy occupied orbital in the box at right: 5. How many nodes does it have? 6. What is the lowest energy of light that titanium ions can absorb? ☐ 1310 kJ/mol ☐ 2865 kJ/mol ☐ 16655 kJ/mol ☐ 20830 kJ/mol 7. What happens when the ion absorbs light of this energy? ☐ an electron is ejected from the 3p subshell ☐ an electron is ejected from the 4s subshell ☐ an electron is excited from the 3p subshell to the 4s subshell ☐ an electron is excited from the 3d subshell to the 4p subshell # of NODES =

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 5 Please rewrite your answer to Q6 here for the ease of the grader: 8. Calculate the wavelength of light that corresponds to your answer to Q6. Remember that the energies were provided in kJ per mole, while wavelength must be calculated from the energy per photon. 9. One advantage of TiO 2 -based sunscreen is that it is not particularly soluble in water. Write a balanced chemical equation showing TiO 2 (s) dissolving into its aqueous ions. 10. Based on the chemical equation you gave above, write an expression for the equilibrium constant (K sp ): 11. The numerical value of K sp for TiO 2 (s) is 2 x 10 – 32 . Calculate the maximum molarity of the titanium ion you could observe in water. 12. How do you expect the solubility of ZnO to compare to that of TiO 2 ? ☐ K sp of ZnO < K sp of TiO 2 ☐ K sp of ZnO > K sp of TiO 2 13. Explain your choice above in terms of Coulomb’s Law.

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 6 Both TiO 2 and ZnO can be obtained via redox reactions between the pure metal and oxygen in the air. Reduction Half-Reaction Eº O 2 (g) + 4e – → 2O 2 – +0.40 V Zn 2+ + 2e – → Zn(s) – 0.76 V Ti 4+ + 4e – → Ti(s) – 1.25 V 14. Use the standard half- cell potentials provided above to calculate ΔEº for the production of both sunscreens: TiO 2 : ZnO: 15. Which sunscreen is more favorable to make? ☐ TiO 2 ☐ ZnO 16. How could you favor the production of TiO 2 or ZnO even more? ☐ Increase the partial pressure of O 2 (g). ☐ Decrease the partial pressure of O 2 (g). ☐ Increase the moles of Zn(s) or Ti(s). ☐ Decrease the moles of Zn(s) or Ti(s). 17. At 298 K, the partial pressure of O 2 (g) in the air is 0.21 atm. Calculate the partial pressure of O 2 (g) when the air is 325 K. 18. It is appropriate to treat O 2 as an ideal gas because: ☐ Oxygen is a very electronegative atom. ☐ The two oxygen atoms have equal electronegativities. ☐ The two oxygen atoms are sp 2 hybridized. ☐ The oxygen molecule has unpaired electrons in the π * MO.

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NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 7 As an alternative to “metal oxides,” some s unscreens utilize “organic” (carbon-based) ingredients such as oxybenzone: 19. Consider the oxygen atom in the C – O – H bond. How would you describe its electron geometry and molecular geometry? ELECTRON GEOMETRY: MOLECULAR GEOMETRY: 20. Consider the carbon atom in the C=O bond. How would you describe its electron geometry and molecular geometry? ELECTRON GEOMETRY: MOLECULAR GEOMETRY: 21. Now describe the orbital that each atom contributes to each bond type. H – O SIGMA BOND C=O SIGMA BOND C=O PI BOND H atom ☐ s ☐ sp ☐ sp 2 ☐ sp 3 ☐ p O atom ☐ s ☐ sp ☐ sp 2 ☐ sp 3 ☐ p C atom ☐ s ☐ sp ☐ sp 2 ☐ sp 3 ☐ p O atom ☐ s ☐ sp ☐ sp 2 ☐ sp 3 ☐ p C atom ☐ s ☐ sp ☐ sp 2 ☐ sp 3 ☐ p O atom ☐ s ☐ sp ☐ sp 2 ☐ sp 3 ☐ p 22. How many of the atoms in oxybenzone have electrons in unhybridized ( “pure”) p orbitals? ☐ 2 ☐ 4 ☐ 6 ☐ 12 ☐ 14 ☐ 16 23. If all of the atomic p orbitals from Q22 combined to form molecular orbitals (MOs), how many of each type of MO would get made? ______ σ ______ σ * ______ π ______ π *

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 8 Compare the structure of oxybenzone to that of benzene: 24. Label which MO diagram corresponds to oxybenzone vs. benzene. LABELS : 25. The absorbance spectra of oxybenzone, benzene, and an unknown molecule (indanthrone) are shown below. Label which is which. 26. What color will indanthrone appear to the eye? ☐ colorless ☐ yellow/orange ☐ blue/purple 27. Would indanthrone be a good sunscreen ingredient? ☐ Yes, it absorbs UV light very strongly. ☐ No, it would dye your skin. ☐ It’s impossible to tell without seeing its MO diagram.

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 9 An imaginary reaction that converts benzene into oxybenzone (plus some H 2 byproduct) is shown below. H – H C – H O – H C – C C – O 436 kJ/mol 415 kJ/mol 464 kJ/mol 345 kJ/mol 350 kJ/mol 28. Estimate the enthalpy of this reaction using the average bond energies. Hint: Focus only on the bonds that get made or broken. The reactant molecules have been drawn in specific locations to help you! 29. Is this reaction endothermic or exothermic? ☐ ☐ 30. All of the reactants are liquids, while the H 2 byproducts are gases. What can you infer about the entropy change of this reaction? ☐ Δ Sº < 0 ☐ Δ Sº = 0 ☐ Δ Sº > 0 31. Based on your answers to Q29 and Q30, is this reaction favorable? ☐ Yes, Δ Gº < 0 at all temperatures. ☐ Yes, Δ Gº > 0 at all temperatures. ☐ No, Δ Gº < 0 at all temperatures. ☐ No, Δ Gº > 0 at all temperatures. ☐ Sometimes, at high enough temperatures. ☐ Sometimes, at low enough temperatures.

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NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 10 32. Fill in the blanks below to explain what happens after oxybenzone absorbs sunlight: The electron eventually _____________ back into the π MO. [ relaxes/excites ] As a result, the potential energy of the system _____________ [ increases/decreases ] and the kinetic energy of the system might _____________. [ increase/decrease ] 33. The absorption of 4 kJ of sunlight raises the temperature of 2 moles of oxybenzone by 11.75 ºC. Based on this information, what is the molar specific heat of oxybenzone (in J mol – 1 ºC – 1 )? 34. For comparison, the specific heat of water is about 75 J mol – 1 ºC – 1 . What is a possible explanation for this? ☐ Water exhibits very strong hydrogen bonding IMFs with itself. ☐ Water has fewer rotational & vibrational degrees of freedom. ☐ Water cannot absorb UV light. 35. Unlike water, oxybenzone is a solid at room temperature. If it gets hot enough, however, it can melt into a liquid. Fill in the blanks below to describe this process. At the melting point, solid-solid IMFs are being _____________. [ broken/made ] As a result, the _____________ energy of the system increases. [ potential/kinetic ] and the temperature of the system therefore _____________. [ increases/decreases/stays the same ]

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 11 36. Dr. S thinks that oxybenzone will dissolve in water, but Deshka thinks it won’t. Provide evidence for both sides by comparing IMF strengths. It could dissolve in water because… It might not dissolve in water because… Unfortunately, once in the water supply, oxybenzone can be toxic. The graph at right shows the titration of a drinking water sample contaminated with oxybenzone: 37. Calculate the number of moles of oxybenzone in the initial sample. 38. The initial sample had a volume of 100 mL. What was the concentration (in M) of oxybenzone in the initial sample? 39. The initial pH was 3.7. What is the value of K a for oxybenzone?

NAME: ___________________________ SSID: _____________________ CHEM 1A Spring 2022 Final Exam 12 Please rewrite your answer to Q39 here for the ease of the grader: 40. The titration experiment will convert all of the oxybenzone into its charged conjugate base form. Complete the Lewis structures below by adding formal charges (if applicable) and lone pairs to the oxygen atoms such that all are fulfilling their “noble gas envy.” Then, indicate which is the correct structure of oxybenzone’s conjugate base . ☐ ☐ ☐ 41. Could you then remove the conjugate base from the water supply? ☐ Yes, if you pour the water over oppositely charged polymer beads. ☐ No, it is now engaged in strong ion-dipole IMFs with water. ☐ Maybe, unless its ID-ID interactions with itself are too strong. 42. Titration with a strong base is not actually a viable decontamination strategy. Instead, you decide to buffer the water supply at a specific pH. Which of the buffer systems below would you choose? ☐ acetic acid + acetate (pK a = 4.8) ☐ hypochlorous acid + hypochlorite (pK a = 7.5) ☐ methylammonium + methylammonia (pK a = 10.6) 43. What is one thing you learned while studying that you didn’t get to show off on the exam? Be detailed and specific — I really want to give you some free points here, so please help me help you ☺

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