General Chemistry I Homework 2

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General Chemistry I Homework 2 Molecular
Motion Liquids Answer
Key

TOM GALE October 2008
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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)
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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(C2H5
SH) 62.1268 g/mol M(N2) 28.02 g/mol tEtSH
tN2 ?(MEtSH/MN2 ) 50 x ?(62.1268 /
28.02) 74.5 s

?
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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
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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
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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