Name ___________________________________ Fourth Midterm Ch 10-11(partial)
R = 0.08206 L·atm/mol·K
1) 1000 L of N2 are prepared at 700 torr and 200°C, then put into a cylinder at 20°C and 30 atm.
Calculate the volume inside the cylinder. (5 pts)
19L
2) 10 L of bromine gas at 0.79 atm and 30°C has a mass of 50 g. Calculate the molecular weight of
bromine. (5 pts)
160. g/mol
3) Calculations similar to problem 2 show that the molecular weight of a particular oxide of sulfur
is 64 g/mol. Calculate its molecular formula. (4 pts)
S1O2 = SO2
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4) An airbag is to be made to inflate to a volume of 10.0 L with nitrogen gas when the following
reaction occurs:
2 AgN3(s)  2 Ag(s) + 3N2(g)
Calculate how many grams of AgN3 must be used to fill the jacket with nitrogen at 3.0 atm and 20°C?
(10 pts)
n(N2) = PV/RT
3 mol (N2) = 2 mol (AgN3)
125 g AgN3
5) Calculate the volume of an ideal gas at 0°C and 1.00 atm if 1.00 mole is used. (3 pts)
22.4 L
6) Calculate the mass of 2 L of CO2 at STP. (4 pts)
m = PVM/RT
4. g CO2
7) Calculate the density of C02(g) at 20°C and 755 torr. (5 pts)
d = PM/RT
1.8 g/L
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8) 1.5 L of hydrogen and 2.0 L of oxygen are mixed together at 3 atm and 200°C and then
reacted. Calculate the mass of gaseous H20 and any other substance will be found under the
original conditions of temperature and pressure. (10 pts)
1) 2 H2 (g) + O2 (g)  2 H2O (g)
2) 2 mol H2 =1 mol O2 = 2 mol H2O or 2 L H2 =1 L O2 = 2 L H2O
3) Calculate limiting reagent : H2
4) 2.1 g H2O
5) 0.154 mol O2 available – 0.058 mol O2 used = 0.096 mol O2 left over = 3.1 g O2
9) A 100 L container has 100 g N2(g), 50.0 g H2(g) and 50.0 g NH3(g), all at 500°C; calculate the
total pressure inside the container. (5 pts)
19.9 atm
10) A 1.0 L container at 10.oC contains 2.0 g He and 2.0 g Ar and has a total pressure of 1.0 atm.
Calculate the partial pressure of the He. (4 pts)
PHe = XHe Ptot
XHe = (2.0 g He / 4.0 g/mol) / ((2.0 g He / 4.0 g/mol) + (2.0 g Ar / 40.0 g/mol)) = 0.50/0.55 = 0.91
Note 1: This problem can also be done the long way by calculating PHe = nHe RT/V if you are
given T and V (which you won’t, the next time you see this problem).
Note 2: The total pressure is actually 11 atm
11) What approximations are made about the gas particles if the gas is to be considered ideal?
(4 pts)
The gas particles have no volume and no forces of attraction (or repulsion) between them.
12) A tank containing HCl and HBr gases developed a leak. Calculate the ratio of the rate of
effusion of HCl to HBr. (5 pts)
E(1)/E(2) = √ (M(2)/M(1))
E(HCl)/E(HBr) = √ (81/36) = 9/6 =1.5
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13) With the aid of a graph show why a gas at higher temperature evaporates more quickly than
one at lower T. Label the axes of the graph, give the two curves, and any important points or
regions on the graph. (4 pts)
See class notes
14) A real gas will act more like an ideal gas at conditions of (high,low) T and (high,low) P. Circle
the two correct words. (4 pts)
I accepted high or low T since there are conditions for each. See text.
15) What are the constants a and b used for in the van der Waals equation? (4 pts)
(P + (n2 a / V2)) (V- nb) = nRT
a: correction for the forces of attraction or repulsion between particles
b: correction for the volumes of the particles
16) Name the intermolecular forces of attraction. (6 pts)
London (or dispersion) forces
Dipole – dipole forces
Hydrogen bonds
17) Name all the intermolecular forces of attraction that are important for the following
molecules.(6 pts)
CH3Cl
London (or dispersion) forces
H3C-O-CH3
Dipole – dipole forces
London (or dispersion) forces
Dipole – dipole forces
NH3 London (or dispersion) forces, Dipole – dipole forces, Hydrogen bonds
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18) What is the name of the force of attraction between the NaCl(aq) ions in solution and the
solvent water molecules. (2 pts)
Ion - Dipole
19) What is the name for the direct conversion of a solid to a gas? (2 pts)
sublimation
20) Circle the substance with the highest heat of vaporization and name the property that makes it
have the highest. (4 pts)
F2
Cl2
Br2
I2
Name: Either London or dispersion forces (microscopic property) or Polarizability (macroscopic
property) can be used to answer.
21) The process of freezing is (exo, endo, non)thermic. Circle the correct prefix. (2 pts)
22) Circle the correct response. The vapor pressure of a substance at its normal boiling point
is____? (2 pts)
1Pa
1torr
1atm “equal to atmospheric pressure”
BONUS (2 pts) Pose and answer a question that you thought would be on this exam but was not.
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