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Unformatted text preview: Chem 105a Exam 2 Last First
Tuesday, February 27, 2007 Last 4 digits Student ID # _ _ _ _
Professor Stephens Lab TA‘s Name
Lab Day & time:
.‘ Major 8‘ 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 nonerasable ink. 4. There are 10 problems on 10 pages. Please count them before you begin. l. (8 pts.) A balloon is ﬁlled with helium at 23°C and 1 atm pressure; the volume is 1.00 L. The balloon rises in the atmosphere to a point where the pressure is 220 torr and the
temperature is 31°C. What is the volume? [R= 0.08206 L.atm/K.mol] ‘1 RV! = Eva
1" T1
ﬂ  laxm Pg ago/7‘0 “M
1“ : 2‘16: K TL '1 JWZK
V2. 3 8‘1".) x (15)
= 'ﬁ ' 7‘ 21:“6 220/760
= $2.sz 2. (12 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? A: E! :: (73377“) x 9.73 = 03‘" “do;
RT (hosting x 231 MM: 7mg =0.3ﬁ7x7o.¢[ = 818.\ 3 _——_d b) What volume will the chlorine gas occupy at STP (0°C, 1 atm)?
\ Mm, ectopic/s 225+ L. @ STP
.'. 0.3T] M5144: OCNP‘a. 8.3q L. 'c) At what temperature will the volume be 3.0 L if the pressure is 800 torr? plv‘ = ()zVZ 2 T2: PZVZ xT .. "T; QV‘ ‘  __ §C . d) At what pressure will the volume be 5.0 L if the temperature is 67°C? PL: PM vs T3: = ("V/7‘s“) X $73 x 3L“
V2 T‘ ' 2‘17 5.0 = 2. ’ll 3 GYM
'= ’Faff‘ 3. (11 pts.) The Hindenburg balloon (didgjble) was ﬁlled with hydxogen gas when it exploded
in 1937. Ifthe volume was 2.0 x105 m at 23°C and 1 atmosphere pressure. how many
grams Hz were in the balloon? u: «3%.? WM"? m cm =mL
8 v= 9.0xlofxmz = ammo L P = \a\"w~ Pvg “KT  '\ = PV/KT . z
..v\= w = 8.23 Hakka“ 0.6826G 132% \ ugh, \Jrz KAI{478.4 Lonexzc ZONS 2.23MB we, at purge. (a $23 X\O vs 2.6% 5'" \‘o.(ax \Qka
I a mono? k3 ll; 4. (10 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 “ :9. R ‘ b) A compound has the empirical formula CHCl. A 256 mL sample at 373°K and 750 torr
contains 0.80 g compound. Calculate the molar mass and derive the molecular formula. &: 0.80 3 e L
23’6 MB“ M = 3.8 x 0%on 1. 37:? = q.“
750(7SQ 5. (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 A: and 0.06 mole fraction CH4. Calculate the partial pressures of N2, Ar and CH4 in the Titan
atmosphere. ‘I PNZ ,, xNZ P 9:: wwu (lend/t6».
= 0.82 ‘LVZZQ
=' \QQO Mr
W Pav ’ xPnr P
: o,\2 ~«Vila ———_—_—_—________’ PC— : XCH~+9
:— Xl‘lz’c 6. (12 pts.) cetylene gas (Csz) is synthesizes by reaction of calcium carbide, CaCz, with water. The‘ alanced equation is: CaCz + H20 —) Ca (0H); + Csz If 5.20 g CaCz are reacted completely, and the acetylene is collected over water, what
volume of gas will be collected at 25°C and 715 torr pressure? Allow for the vapor
pressure of water, which is 25°is 23.8 torr. chz +2wzc> ——9 CaiWriziC’ziiz,
ladle. (‘0th —> \ Mm. C7,“; * (oq..\
O.o8\ who Cat; —, Ohm wmc (an; 52o? Cpl; = 3'2°_.,u,og\ MM: CatCL Pv= ngT V= “WI? ~08 Q 1K 3
V: (108‘ 3‘ 0. Z (a =2.\8L C 6q\.2../’76o 7 7. (8 pts.) a) A system takes in heat q and does work w on its surroundings. Its energy changes by
AE. What is the equation relating AE to q and w? mintW. b) A system expands in volume by AV against a constant external pressure P. What is
the equation for the work done by the system on its surroundings, w? w: PAV c) Write the equation deﬁning the enthalpy, H, of a system. H=E+PV d) A system takes in heat q at constant volume. What is the change in energy, AE? ABE—:3, 8. (11 pts.) Consider the reaction in which magnesium metal burns in oxygen to
form magnesium oxide: 2Mg(s) + 02(g) ——) 2Mg0(s) AH = 4204“
a) Is this reaction exothermic or endothemiic? QXQ‘rhefMQc. b) When carried out at constant pressure, how much heat is given out when 2.4 g of
magnesium is burnt? c) If 96.0 k] of heat are given out, how many grams of MgO are produced? [26¢ \cT 1L7. «61»; MO " " 96$ ((3— “ = 0"“ “KM =(043’q x L\.0.?I ) 5. NO 3 (9475 d) How many k] of heat are required to cause 7.50 g MgO to decompose to magnesium
and oxygen? guﬁo > 2M3 +6; AW: HzmncT
7.52:5 MS a O.\8b Mu, M50 OAXB Mn» M30 bum!“ “‘1 0A“ k\1¢‘l' =_ hsz M
Z 9. (11 pts.) From the following enthalpies of reaction: H2(g) + F2(g) —> 2HF(g) AH=537kJ
C(s) + 2F2(g) —> CF4(g) AH=680kJ
2C(s) + 2H2(g) 4‘ C2H4(g) AH=+52.3 kJ calculate AH for the reaction of ethylene with F2: C2H4(g) + 6F2(g) —> 2CF4(g) + 4HF(g)
cw“, ——v 2C, + z»; A“ = ‘32.: k3
~—> ZCE, AH: ?GQ\C.~T any» 2% —> «4*? A“ ‘ "WWW
/___,____._. Cut“, 4 (OF; #2021 4— K—HF
A“ = (—33.2 —\3(m —N1u)tT ﬂ/M 10. (11 pts.) The space shuttle orbiter utilizes the oxidation of methyl hydrazine by dinitrogen
tetroxide for propulsion: 4N2H3CH3(I) + 5N204(l) ——> 12H20(g) + 9N2(g) + 4C02(g) Calculate AH° for this reaction using the heats of formation given in the attached Table A“ = §“PLAH+ )p" g “P (AHfL‘ = I2. (A“§(“2°,3)) + ‘1 Misﬁt/2,3»
4' 44A“; ((023 3)
—— SFCAHP (Nzotr’ )
._,+ (ARC? (NZHXCA’B’ ﬂ )) IOW'H I‘m? ’1 90800 = H .INVLSNOZ) SVD ’IVSHEIAINH 51mm Ell11:10 ETIEIVJ. DIGOIHEId Zumdahl Agnendix 4 AH°,(kJ ml“) at 298.15K
CO(g) 110.5
002(2) 3.5
75 ii ...
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