YOUR NAME (last name, first name)...answers... FALL 2011 MIDTERM. (Total number of pages = 10) (Total points = 110) (Total time = 110 minutes)

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1 CHEM 14A Instructor: Dr. Laurence Lavelle YOUR NAME (last name, first name)...answers... STUDENT ID#... FALL 2011 MIDTERM (Total number of pages = 10) (Total points = 110) (Total time = 110 minutes) **Carefully remove the last two pages: Constants and Formulas, and Periodic Table.** YOUR DISCUSSION SECTION... YOUR TA s NAME.. WRITE IN PEN (Show all your work on this paper, check units and significant figures.) Box your final answer. Good Luck 1

2 Do not write on this page. QUESTION SCORE TOTAL (max 110) 2

3 Q1A. A compound found in tea is believed to be a stimulant. Its molar mass is g.mol and has a mass percentage composition of 49.47% C, 5.18% H, 28.86% N, and 16.49% O. What is its molecular formula? (10pt) Obtain actual masses of each element in a mole of the compound g/mol x = g C g/mol x = g H g/mol x = g N g/mol x = g O Then calculate # of moles of each element g Mol of C atoms = 12.01g mol = 8.00 mol g Mol of H atoms = = 9.98 mol g mol g Mol of N atoms = = 4.00 mol 14.01g mol g Mol of O atoms = = 2.00 mol g mol Molecular formula ofthis compound is C 8 H 10 N 4 O 2 2pt Q1B. Calculate the volume needed to make 2.00 L of a M K 2 CrO 4 from 1.75 M K 2 CrO 4 stock solution. (4pt) 2pt 2pt 3

4 Q2. Mix some sodium bicarbonate with lemon juice and cool frothy bubbles result: NaHCO 3(s) + C 6 H 8 O 7(aq) Na 3 C 6 H 5 O 7(aq) + H 2 O (l) + CO 2(g) Will all the sodium bicarbonate (NaHCO 3 ) be used up if moles of it reacts with 50.0 ml of a M solution of juice. If not, how many grams will be left over? (12pt) Balance the chemical equation: 3NaHCO 3(s) + C 6 H 8 O 7(aq) Na 3 C 6 H 5 O 7(aq) + 3H 2 O (l) + 3CO 2(g) 5pt Find moles of juice (C 6 H 8 O 7 ): Moles = Molarity x Volume (L) = M x L = mol C 6 H 8 O 7 3:1 mole ratio between NaHCO 3 and C 6 H 8 O 7 means mol C 6 H 8 O 7 will react with mol NaHCO 3. There is an excess of NaHCO 3 since there is mol mol = mol NaHCO 3 remaining Molar mass NaHCO 3 : g.mol g NaHCO mol NaHCO 3 x 1mol NaHCO = 0.21 g NaHCO 3 remaining 2pt 3 4

5 Q3A. A newly discovered metal is determined to have a work function of 5.98 x 10 8 J. 6 If the ejected electron is traveling at m s, what is the wavelength of the incident photon? (12pt) hc 2pt 2pt 2pt ( kg)( m s ) 18 mv J J 2pt 2pt 2pt 34 8 hc ( J s )( m s ) m 25.4 nm 8 E J Q3B. Which of these has the longer de Broglie wavelength? A proton (mass = x kg) moving with velocity of 3.82 x10 7 m.s or an electron (mass = x kg) moving with velocity of 1.20 x 10 7 m.s? Show your work. (6pt) = h/p = h/(mv) Proton: = (6.626 x J.s)/((1.672 x kg)(3.82 x 10 7 m.s )) = 1.04 x 10 4 m Electron: = (6.626 x J.s)/((9.109 x kg)(1.20 x 10 7 m.s )) = 6.06 x 10 1 m The electron has a longer de Broglie wavelength. 5

6 Q4A. If you want to listen to an FM radio station that is broadcasting with radio wave of wavelength 3.28 m, what frequency should you be tuning in to? (Give your answer in MHz.) 8 1 c m s 6 1 v s 91.5 MHz λ 3.28 m (4pt) Q4B. Heating Copper (I) Chloride to about 1200 C results in blue fireworks. The hot compound emits blue light with energy of 4.40 x 10 9 J per photon. What is the wavelength of the blue light in nanometers? (4pt) ΔE hν and c v λ hc ( J s)( m s ) -7 λ m 450.nm 19 ΔE J Q4C. What is the minimum uncertainty in the speed of an electron confined to within the diameter of a tin atom which has a diameter of 145 pm? (4pt) ΔpΔx h 4π Δp m e Δ v Δx 145pm m Δv h kg m s m s πmeΔx 4(3142)( kg)( m) 6

7 Q5. Two structural isomers of disulfur difluoride, S 2 F 2, are known. In each the two S atoms are bonded to each other. In one isomer each of the S atoms is bonded to an F atom. In the other isomer, both F atoms are attached to one of the S atoms. Draw each isomer and a resonance structure for each isomer, and determine for each isomer which structure is favored and why. (16pt) +1 F S S F F S S F favored isomer 1 F F +1 S S F F S S isomer 2 favored 3pt for each structure (12pt) for correctly identifying lower energy isomer (2pt) 2pt for correct formal charge calculations or for saying lower FC. 7

8 Q6A. Write the ground-state electron-configuration for Sn 2+ [Kr]4d 10 5s 2 Dark red copper(i)oxide (Cu 2 O) forms when copper metal is oxidized by air. Write the ground-state electron-configuration for Cu + [Ar]3d 10 (2pt) (2pt) Q6B. Which compound is more ionic? (4pt) (a) Cl 2 O or Na 2 O (b) InCl 3 or SbCl 3 (c) LiH or HCl (d) MgCl 2 or PCl 3 (a) Na 2 O; (b) InCl 3 ; (c) LiH; (d) MgCl 2 Q6C. The structure for the amino acid tryptophan is given below with lone pairs omitted. Also some of the hydrogen atoms attached to carbon atoms are omitted. (8pt) What is the hybridization of the carbon atoms in the ring structure? sp 2 What is the hybridization of the nitrogen atom pointing towards you? sp 3 What is the hybridization of the last oxygen atom on the far right? sp 3 What is the hybridization of the carbon atom that is connected to the two oxygen atoms? sp 2 8

9 Q7. Draw the lowest energy Lewis structure and determine the following information for the species given. (a) Dichloromethanol: HOCHCl 2 (6pt) VSEPR formula: AX 4 molecular shape: tetrahedral hybridization of central atom: sp 3 polar or non-polar: polar give one bond angle: o or ~109.5 o (b) Phosphoric acid, H 3 PO 4, can give off three protons to form the phosphate anion, PO 3-4. Answer the following questions for PO 3-4. (5pt) VSEPR formula: AX 4 molecular shape: tetrahedral give one bond angle: o -2pt for incorrect structure. - for each incorrect answer. 9

10 Q8. Draw the lowest energy Lewis structure and determine the following information for the species given. (a) Dimethyl sulfoxide (DMSO), (CH 3 ) 2 SO, is an important solvent for polar and nonpolar compounds. Answer the following questions for DMSO. (6pt) VSEPR formula: AX 3 E molecular shape: trigonal pyramidal polar or non-polar: polar give one bond angle: less than o (b) Nitric acid, HNO 3, can give off one proton to form the nitrate anion, NO 3 -. Answer the following questions for NO 3 -. (5pt) VSEPR formula: AX 3 molecular shape: trigonal planar hybridization of central atom: sp 2 give one bond angle: 120 o -2pt for incorrect structure. - for each incorrect answer. 10

YOUR NAME (last name, first name)...answers... FALL 2018 MIDTERM. (Total number of pages = 10) (Total points = 110) (Total time = 110 minutes)

CHEM 14A Instructor: Dr. Laurence Lavelle YOUR NAME (last name, first name)...answers... STUDENT ID#... FALL 2018 MIDTERM (Total number of pages = 10) (Total points = 110) (Total time = 110 minutes) **Carefully