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************************************************* General Chemistry I Homework 2

************************************************* General Chemistry I Homework 2 Molecular Motion & Liquids Answer Key *************************************************. TOM GALE October 2008.

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************************************************* General Chemistry I Homework 2

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  1. ************************************************* • General Chemistry I Homework 2 • Molecular Motion & Liquids • Answer Key • ************************************************* TOM GALE October 2008

  2. Q1: The average speed of molecules of a toxic gas emitted in a car exhaust at 158 ºC (to 3 s.f.) was found to be 0.62 km/s. Deduce the molar mass of the gas and hence its identity. A: CO is a gas. Molar mass = 28.01 Da using urms = (3RT/M) urms is the ‘root mean square’ velocity = 620 m/s (SI units) M = molar mass T = 431 K 620 = (3 x 8.314 x 431 / M) M = 0.02797 kg/mol (SI units) = 28.0 g/mol gas is carbon monoxide (CO, Mr = 28.01 g/mol)

  3. Q2: A pin-sized hole is made in a container of the smelly gas ethanethiol (C2H5SH) the vapour effuses out. How long did the gas take to effuse, given that a similar molar quantity of diatomic nitrogen took 50 seconds? tA = MA tB = MB M(C2H5SH) = 62.1268 g/mol M(N2) = 28.02 g/mol tEtSH =tN2 (MEtSH/MN2 ) = 50 x (62.1268 / 28.02) = 74.5 s 

  4. Q3:Look at the boiling points of the following substances and look up their structures if required: a) which is a gas at room temperature? b) which of the liquids is not a compound? c) of the liquids, which is most volatile? d) which substance has the lowest vapour pressure? e) which of the substances has intermolecular hydrogen bond (look up structures if required)? f) Which of the substance(s) consist(s) of non-polar molecules held together by van der Waals forces only? g) Which of the substance(s) is held together by metallic bonding? h) Which of the substance(s) are polar but not hydrogen bonded? acetamide liquid boiling point ºC ethyl formate 54 ethyl bromide 38 ethanol 78 ethylene -104 acetamide 222 gallium 2205 ethyl formate ethylene ethyl bromide gallium ethyl bromide gallium ethanol, acetamide ethylene gallium ethyl formate, ethyl bromide

  5. Q4. Look at the phase diagram for carbon: a) At 107 Pa pressure what is the sublimation temperature of graphite? b) What happens if the same heating is carried out at 5 x 106 mmHg pressure? c) What happens to hot graphite at 3200K and 1010 Pa when it is cooled to room temperature? d) At 2500 K what minimum pressure (atm) is required to turn graphite into diamond? 4100K graphite melts changes to diamond 100,000 atm

  6. Q5: Consider three nitrogen compounds of similar molar mass, composed exclusively of each of the three molecules shown below. N2H4 (hydrazine) a) which is hydrogen bonded? b) which has dipole-dipole interactions but is not hydrogen bonded? c) which has the lowest boiling point? Why? b.p. 26 ºC HCN N2(no dipole moment so the only intermolecular forces are weak van der Waals’) b.p. 114 ºC b.p. -196 ºC

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