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exam_2_spring_2006

exam_2_spring_2006 - Chem 105a Exam 2 Last First Initial...

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Unformatted text preview: Chem 105a Exam 2 Last First Initial Tuesday, February 28, 2006 Last 4 digits Student ID # Professor Stephens Lab TA's Name Lab Day & time: .Ifl Major fl Total _—— W Please Sign Below: I certify that I have observed all the rules of Academic Integrity while taking this examination. Signature: INSTRUCTIONS 1. You must show your work to receive credit. 2. If necessary, please continue your solutions on the back of the preceding page (facing you). 3. YOU MUST Use non-erasable ink. 4. There are 12 problems on12 pages. Please count them before you begin. The Gas Constant R is 0.08206 (L.atm/mole°K) Exam 2 1. (8 pts.) Citric acid (lemon juice) has the molecular formula C6H807. A 0.250 g sample of citric acid dissolved in 25.0 mL water requires 37.2 mL of 0.105 M NaOH solution for complete neutralization. How many acidic H atoms (i.e., H atoms that ionize in aqueous solution) per molecule are there in citric acid? (it) 0.1503 (.ng 07 = 023,0 MOM ‘92 . \z -3 ,_ 2 '30 &\Q MGM (AVIS; qugx \it') 371mg mms‘m Mam Jaw.“ CUM-mas 4 3.9\ $\m Mn», N355“- \l“) 3.9\ X\\.3 MAM—e0 mm NMMZQ, .. + ‘ 3.9\ X\Q 3 MQf/g \‘*JQ NM; 2 (mu-H36" a 1“; Q Q») (($367 ——» pal—E? o + BL”- o7] l-301< \Q—J MM (“\Yn‘c Out;& ~9E\.3°1\\0’3 x x J (May; HJQ 7c: 2 Mfg M 2 QCXJCC H 03mm .5“, (“Aria O‘mZK . “— 4. 2. (8 pts.) Sulfuric acid spilled on a lab bench can be neutralized by adding solid sodium bicarbonate (NaHCOs). The reaction products are sodium sulfate, carbon dioxide and water. If 35 mL of 6.0 M sulfuric acid is spilled, what is the minimum mass of sodium bicarbonate which must be added to neutralize the acid? \ Mm. “260+ Nada um. 2, may, mum? ... O.2_\ vwfl-eo “-25% NW Quie‘ NM NQWKQJ, -—- Funm Q.“ 2 M010 \V2~\'\' Q $3 = QALZ‘k 34.6I 35:35 WWCQZ lo Flux/WAC 4WD NWMA Hi“ M‘V‘hl (XGZQ _ 3. (10 pts.) "Chromium 6" (Le. chromium in the oxidation state +6) is featured in recent news articles and in the film Erin Brockovich as an important environmental pollutant which may affect human health. What are the oxidation states of Cr in the following compounds? a) 003 +2 QV3+)CCJL—‘)z b) CrOze + - -l +C> (CVQ: )(OZ )2(F )1 c) Na3CrO4 +g LNJ‘)?’ [Gs-{KEY 2+ ]3.. d) (NH4)2Cr207 + c, [@VQHHA 3+ [CC"+(§)L <6“ )7 J 4. (12 pts.) What are the oxidation states of each of the atoms in the following compounds? 21) N03' 0 —2’ [WM—Hf )3 3.. b) H202 H +i 0+”; (5‘)z A i Jr '3 :5: RN W A3 m +7 (@“Xo’zkfj Q —- 2. d) CH3OH 5. (8 pts.) In a blast furnace, iron oxide (Fe203) is converted to iron by reaction with coke (elemental carbon). Assuming that the C is converted entirely to C02, a) write a balanced equation for the reaction QRL63+ :31 —> 4-9 +§CQ2 b) define the oxidation states of Fe in (i) Fe203 (ii) iron + 3 o and of C in (i) coke (ii) C02 0 +(+. c) write balanced equations for the half-reactions involving (i) oxidation ‘fi- C—Ll—e -> C Q—i— Q02 ~WQ —% C02 (ii) reduction .S-€3++ 3e” —-> FL 6. (10 pts.) Metallic zinc dissolves in nitric acid to form a solution of zinc nitrate and, simultaneously, nitrous oxide gas (N20). a) What are the oxidation states of Zn in (i) Metallic zinc O + 2_ (Zniz )( N002 b) What are the oxidation states of N in (i) nitric acid - +—€' H'L [N+{'(O )3] (iii) nitrous oxide —Z +\ H Q! A. (o ) c) Write the balanced equations for the half-reactions involving (ii) zinc nitrate (i) oxidation 2+ Zn. “2Q —? Zn (ii) reduction. , \+ JNb++8Q- “‘7 2N ,- 2“ _. ‘ —\- ‘— 36 2NDJ + to“ +82 —> NLO 4- 3/120 d) Combine your half—reactions in part c) to give a balanced equation for the overall reaction. 4..er 4" 2ND: ‘l" (QR—‘— —~> N D +Kl~Zv~2+ “‘3sz l1 42w lOH-Nog —-> g Zv~(N°3)2_ ““10 +9419 7. (8 pts.) Chlorine gas is used to sterilize swimming pools. Suppose a particular sample of chlorine occupies 9.73 L at 755 torr and 24°C. a) How many grams of chlorine gas are there? In: 9" =C’ 5/760) X 9' 73 = Q3q7 woe/J (3.082% x Zq7 MN = 75.0. #3 —.- 03°“! x 7&3 = 28. l 3 b) What volume will the chlorine gas occupy at STP? \ «MILL octupwo 224+ L @ 5T? Q.3<‘7 «who Mij 8.3‘] \— c) At what temperature will the volume be 3.0 L if the pressure is 800 torr? fiV\ _: PLVL .'. 1—2.: VZVZ $T‘ T, T1 «Vi -.-. 8m” 30 $737 = 97°\<-—\7g C» ‘75’.>""()'73 'l x \9 .1 w x w .g 0 ‘72-:— fivt 7g T2. —__ \[2 T! 5:0 2‘7 8 8. (6 pts.) The Hindenburg balloon (dirigible) was filled with hydrogen gas when it exploded in 1937. If the volume was 2.0 x105 m3 at 23°C and 1 atmosphere pressure, how many grams H2 were in the balloon? '5' \{Mg V = 2.Q*\Q M3 \L = \Q3 0.53 \m3 =— mkc‘mz = \QSL " ‘8 V: 2.0% \Q3 *\Q$ = 2.0?“- \Q l_ T'; 237+ng = 23x0°K Q3 ‘Ox‘fiw 0V: “(T “3 W/QT ‘8 k 9. (8 pts.) The molar mass of a gas is related to its density. a) What is the equation for the molar mass (M) in terms of the density (d)? M=dfl 1? b) A compound has the empirical formula CHCl. A 256 mL sample at 373°K and 750 ton contains 0.80 g compound. Calculate the molar mass and derive the molecular formula. &=. 6.86 ‘ 3.‘?j /" 2519* lo”: M: ?.\gx (3.0815(0 1273 a 9.," 75‘“ I765 10 10. (6 pts.) Air bags in automobiles are inflated using the explosive decomposition of sodium azide, NaN3, which liberates gaseous nitrogen. The (unbalanced) reaction is: NaN3(S) -* Na(S) + N2(g) Calculate the mass of sodium azide required to produce 100 L of gaseous nitrogen at STP. SW: 0°C=273°K M \R’M- V:— loot... \VV:—V\QT V\= PV/QT‘ lme \SQL. {5 “z = unwio WUJU/a N2, 0.082% $273 M2 ll 11. (10 pts.) An organic compound containing N, whose formula is CxHyNz, is passed over heated CuO. The products of the reaction are nitrogen gas, carbon dioxide gas and steam. The product gas is passed through a KOH solution to remove carbon dioxide and collected over water. In a given experiment 0.253 g compound give 31.8 mL of wet nitrogen gas at 25°C and 726 torr. Given that the vapor pressure of water at 25°C is 23.8 torr, calculate the mass percentage of N in the original compound. [This is the Dumas method for analyzing the N content of organic compounds] C)‘“-3Nz' +CV~Q ’9 NL+ (Q2 i. “fa-Q 3L? ML we.» N1 84/; @ ZK‘QITZQ-lwv 3\.9mu NJ “‘2. @ 292, M5 726—23.8 = 7QZ.Z_‘\‘UW‘ n 2 PN \l __ 1°21 3L9 NZ 2. 79¢ )( l‘Q—u—Q ) 0.081og wk 25}? LZQ ‘K \Q—a MM 1 = I.2\\"~\Q- 7‘ 28.02. = 33.‘° 7K \Q-g & NZ. -3 0.2535 <3,»sz cm“. mum 3N2 . ~3 £3.75: 2 8.32., 12 12. (6 pts.) From data gathered by Voyager 1, scientists have estimated the composition of the atmosphere of Titan, Saturn's largest moon. The total atmospheric pressure on Titan is 1220 torr. The atmospheric composition is: 0.82 mole fraction N2, 0.12 mole fraction Ar and 0.06 mole fraction CH4. Calculate the partial pressures of N2, Ar and CH4 in the Titan atmosphere. 1’ N2 XNLP x = mm, “WWW = Q32. 1s V2.20 = \OQQ JNN‘ _______________._..— EA; MP 0.\2,~I- \220 = “Ho *wrc- PC“Lt - 7km»? ‘ 0.Qg+\220 ‘-"- 73 ‘l'nrv- \K\oo -\- “Ho +73 3 / \Z\9 c3, lZZQ law. ...
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