Chem1000A
Spring 2005
Assignment 9 - Solutions
Two or three questions will be marked from each assignment.
DUE ON March 29, 2005 (Tuesday) 1:00 PM
To be dropped off at my office (E860)
1. The atmospheric pressure at the surface of Venus is 90.8 atm. The Venusian atmosphere is 96.5%
CO2 and 3.5% N2 by volume, with small amounts of other gases also present. Compute the mole
fraction and partial pressure of N2 in the atmosphere of Venus.
At constant p and T, V is proportional to n. Therefore, volume percent and mole percent are the same
for ideal gases.
Mole percent: 96.5 % CO2 ; 3.5 % N2
X(CO2) = 96.5 %/100% = 0.965
X(N2) = 3.5 %/100 % = 0.035
Partial pressure:
p(N2) = X(N2) × ptot = 0.035 × 90.8 atm = 3.2 atm
2. You combine 0.0650 mol of Ar, 0.0250 mol of N2, and 0.0960 mol of CH4 in a 5.000 L flask at 25
°C. Calculate the partial pressures of Ar, N2, and CH4, and the total pressure inside the flask.
X(Ar) = 0.0650 mol/(0.0650 mol + 0.0250 mol + 0.0960 mol) = 0.0650/0.1860 = 0.349
X(N2) = 0.0250 mol/(0.0650 mol + 0.0250 mol + 0.0960 mol) = 0.0050/0.0950 = 0.134
X(CH4) = 0.0960 mol/(0.0650 mol + 0.0250 mol + 0.0960 mol) = 0.0700/0.0950 = 0.516
Total pressure: pV=nRT
ntot = 0.0650 mol + 0.0250 mol + 0.0960 mol = 0.1860
T = 25 °C = (25 + 273.15)K = 298 K
V = 5.000 L ×(1 m3/1000 L ) =0.005000 m3
ptot = ntotRT /V = 0.1860 mol × 8.3145 J mol-1 K-1 ×298 K/0.00500 m3=92200 J/m3
= 92200 kg m2 s-2m-3 = 92200 kg s-2m-1 =92200 Pa
Partial pressures: pi = Xi× ptot
p(Ar) = 0.349 × 92200Pa = 32200 Pa
p(N2) = 0.134 × 92200 Pa = 12400 Pa
p(CH4) = 0.516 × 92200 Pa = 47600 Pa
3. Which statement(s) is/are correct about a gas at thermal equilibrium?
(a) All gas molecules have the same temperature. Incorrect; you cannot define a temperature for
specific molecules.
(b) All gas molecules travel with the same speed. Incorrect; the speeds of molecules have a MaxwellBoltzmann distribution.
(c) The temperatures of the gas molecules have a Maxwell-Boltzmann distribution. Incorrect; one
specific temperature of a gas is a consequence of a Maxwell-Boltzmann distribution of speeds.
(d) The speeds of the gas molecules have a Maxwell-Boltzmann distribution. Correct.
4. You fill 1.000 kg of SO2 into a 1L cylinder at 25 °C. (a) What is the pressure in Pa inside the
cylinder considering the SO2 as an ideal gas? (b) What is the pressure in Pa inside the cylinder using
the van-der-Waals equation to account for the real behaviour of SO2? (SO2: a = 6.714 atm L2 mol-2;
0.05636 L mol-1) – For (b), you can use non-SI units in your calculations.
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M(SO2) = 64.0648 g/mol
n(SO2) = 1000. g/(64.0648 g/mol) = 15.61 g
pV = nRT; for SO2: V = 1 L = 0.001 m3; T = (25 + 273.15)K = 298 K;
(a)
p =nRT/V = 15.61 mol × 8.3145 J K-1 mol-1 × 298 K/0.001 m3 = 38,680,000 J m-3 = 38,680,000 (kg
m2 s-2) m-3= 38,680,000 kg m-1 s-2 = 38,680,000 Pa
(b)
[p+a(n/V)2][V-bn]=nRT
p = nRT/[V-bn] - a(n/V)2
= 15.61 mol × 8.3145 J K-1 mol-1 × 298 K/[1L - 0.05636 L mol-1×15.61 mol] - 6.714 atm L2 mol-2
×(15.61 mol/1L)2 =320,000 atm × (101325 Pa/1 atm) = 32,400,000,000 Pa
Under these extreme conditions (high pressures), the ideal gas law cannot be applied.
5. Specify the interparticular forces present in the following compounds:
(a) sodium fluoride, NaF
This compounds consists of Na+ cations and F2- anions. Both ions have no dipole moments.
Intermolecular forces:
Ion-Ion interactions
Ion-induced dipole interactions
London dispersion forces
(b) hydrogen sulfide, H2S
This molecule has a bent molecular geometry and is polar, i.e., it has a dipole moment.
Intermolecular forces:
Dipole-dipole interactions, more specifically: hydrogen-bonding interactions
Dipole-induced dipole interactions
London dispersion forces
(c) sulfur hexafluoride, SF6
This molecule has an octahedral molecular geometry and is non-polar, i.e., it has no dipole moment.
Intermolecular forces:
London dispersion forces
(d) potassium perchlorate, KClO4
This compounds consists of K+ cations and ClO4- anions. Both ions have no dipole moments – ClO4is tetrahedral.
Intermolecular forces:
Ion-Ion interactions
Ion-induced dipole interactions
London dispersion forces
(e) XeO4
This molecule has an tetrahedral molecular geometry and is non-polar, i.e., it has no dipole moment.
Intermolecular forces:
London dispersion forces
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(f) NO2F
This molecule has a trigonal pyramidal molecular geometry and is polar, i.e., it has a dipole moment.
Intermolecular forces:
Dipole-dipole interactions
Dipole-induced dipole interactions
London dispersion forces
(g) SeF4
This molecule has a seesaw molecular geometry and is polar, i.e., it has a dipole moment.
Intermolecular forces:
Dipole-dipole interactions
Dipole-induced dipole interactions
London dispersion forces
(h) OH2
This molecule has a bent molecular geometry and is polar, i.e., it has a dipole moment.
Intermolecular forces:
Dipole-dipole interactions, more specifically: hydrogen-bonding interactions
Dipole-induced dipole interactions
London dispersion forces
(i) OF2
This molecule has a bent molecular geometry and is polar, i.e., it has a dipole moment.
Intermolecular forces:
Dipole-dipole interactions
Dipole-induced dipole interactions
London dispersion forces
In order to answer this question you have to draw the Lewis structures and determine the VSEPR
molecular geometries.
6. What interparticular forces are present in a solution of NaF in H2O?
You have to consider three particles in this solutions: Na+, F-, and H2O
You have the following interparticular interactions:
Ion-ion between Na+ and FIon-dipole interactions between Na+ and H2O, and F- and H2O.
Ion-induced dipole interactions between one ion and any other particle
Dipole-dipole between two water molecules
Dipole-induced dipole between water and any other particle
London dispersion forces between any two particles
7. Comparing the intermolecular forces present in pure CCl4 and in pure CI4. Which of these two
compounds has a higher vapour pressure at room temperature and which one has a higher boiling
point? One of the compounds is a liquid and the other is a solid. Identify the solid among the two
tetrahalides. Provide explanations for your answers.
Both tetrahalides are tetrahedral, non- polar compounds.
The intermolecular forces present are London dispersion forces
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CCl4 is smaller than CI4. Therefore, CI4 has a much higher polarizibility and has much stronger
London dispersion forces. As a consequence, CI4 has a lower vapour pressure and a higher boiling
point than CCl4.
The difference in strengths of the intermolecular forces results in different states of matter for these
two compounds. Carbon tetrachloride is a gas and carbon tetraiodide is a solid.
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