125pex1sol - Copyright RJZ 1/27/12 1 no unauthorized use...

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Copyright RJZ 1/27/12 1 no unauthorized use allowed Chemistry 125 - W12 Solutions for Practice Midterm 1 This material is copyrighted. Any use or reproduction is not allowed except with the expressed written permission of Dr. Zellmer. If you are taking Chem 125 you are allowed to print one copy for your own use during the quarter you are taking Chem 125 with Dr. Zellmer. You are not allowed to disseminate this material to anyone else during the quarter or in the future.
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Copyright RJZ 1/27/12 2 no unauthorized use allowed 1) P &V change at constant T - Use Boyle’s Law P 2 V 2 = P 1 V 1 or P f V f = P i V i P 1 = 1456 mm Hg P 2 = ? V 1 = 2.00 L V 2 = (2.00 + 5.00) = 7.00 L P 1 V 1 V 1 2.00 L P 2 = --------- = (-----) P 1 = (---------) 1456 mm Hg = 416 mm Hg V 2 V 2 7.00 L Check: If V inc, P dec and it did. D 2) Use Combined Gas Law : P f V f P i V i ------- = ------- T f T i P f = 2P i V f = 1/3 V i (Vol dec to 1/3 its initial value) P f V f P f V f T f = ------- T i = (-----)(-----) T i P i V i P i V i 2 P i 1/3 V i T f = (-------) (---------) T i = (2/3)T i P i V i E
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Copyright RJZ 1/27/12 3 no unauthorized use allowed 3) To get the pressure determine the moles of 1 hydrogen atom and then use IGL (PV=nRT) to get the pressure. 1 mol Hatoms ? mol H = 1 H atom × -------------------------- = 1.66112 x 10 ! 24 mol Hatoms 6.02 x 10 23 H atoms nRT (1.66112 x 10 ! 24 mol)(0.08206 L C atm/mol C K) (10 K) P = ------- = --------------------------------------------------------------------- V (1000 L) = 1.36312 x 10 ! 27 atm 760 torr P = 1.36312 x 10 ! 27 atm x ------------ = 1.03597 x 10 ! 24 atm = 1.0 x 10 ! 24 atm 1 atm A 4) According to Avogadro’s Law, equal volumes of gases under the same P & T conditions contain the same # moles (same # particles). V 2 V 1 n 2 ----- = ----- so V 2 = ---- V 1 n 2 n 1 n 1 V 1 = 389 mL V 2 = ? n 1 = 0.0188 mol n 2 = 0.0243 mol n 2 0.0243 mol V 2 = ---- V 1 = ---------------- (389 mL) n 1 0.0188 mol = 502.80 mL = 503 mL E
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Copyright RJZ 1/27/12 4 no unauthorized use allowed 5) Determine identity of x in SF x . If know MW (molar mass or molecular wt.) of SF x can find x. Have mass of 1.00 g, need moles. (Remember, molar mass = g/mol, same as MW.) 1 atm P = (745 mmHg × --------------- ) = 0.98 0 2 atm; T = 75°C + 273.15 = 34 8 .15 K 760 mmHg PV (0.98 0 2 atm) (0.199 L) n SFx = ----- = -------------------------------------------- = 6.8 2 80 x 10 ! 3 mol RT (0.08206 L C atm/mol C K) (34 8 .15 K) 0.738 g = ------------------------ = 10 8 .08 g/mol or 10 8 .08 amu 6.8 2 80 x 10 ! 3 mol MW = 1 AW S + x AW F 10 8 .08 = 1 (32.06) + x (19.00) 7 6 .02 = x (19.00) x = 4.00 SF 4 D
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Copyright RJZ 1/27/12 5 no unauthorized use allowed 6) Use Dalton’s Law of Partial Pressures, P Total = P 1 + P 2 + P 3 + CCC Each gas acts independently of the other gases and each follows the IGL as does P Tot . P T = P O2 + P He P O2 = P T & P He = 3.0 & 2.80 = 0.20 atm Want mass of He - find moles first PV (0.20 atm) (1.0 L) n = ------ = -------------------------------------------- = 0.008 3 139 mol O 2 RT (0.08206 L C atm/mol C K) (293.15 K) 32.00 g O 2 ? g O 2 = 0.008 3 139 mol O 2 × --------------- = 0.2 6 60 g = 0.27 g O 2 1 mol O 2 A 7) The density is directly proportional to the molar mass (molecular or atomic weight). Therefore, the gas with the greatest molar mass (MW or AW) should have the greatest density (since P, V & T same for all). The MW are listed below, as are the densities (which you didn’t need to calculate). RT P = (------) D => D = (------) (in g/L) P RT (a) (b) (c) (d) (e) Ne (20.18) CH 4 (16.04) F 2 (38.00) NH 3 (17.03) C 2 H 4 (28.05) 0.900 0.715 1.69 0.759 1.25 C
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Copyright RJZ 1/27/12 6 no unauthorized use allowed 8) The rate of effusion is directly proportional to the molecular speed which is inversely proportional to the square root of the molar mass. Thus, lighter particles have a faster average velocity and faster rate of effusion.
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This note was uploaded on 02/19/2012 for the course CHEM 125 taught by Professor Zellmer during the Winter '08 term at Ohio State.

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125pex1sol - Copyright RJZ 1/27/12 1 no unauthorized use...

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