THE COPPERBELT UNIVERSITY
SCHOOL OF MATHEMATICS AND NATURAL SCIENCES
CHEMISTRY DEPARTMENT
CH 110 TUTORIAL: GASES
Question 1:
a) Covert the following pressures to the given units.
i.
0.357 atm to Pascals.
ii.
6.6 x10-2 torr to atmospheres.
iii.
147.2 kPa to mmHg.
b) Define the following terms:
iv.
Pressure
v.
Partial pressure
vi.
Mole fraction
vii.
Root-mean-square-speed
viii.
Effusion
c) Give mathematical and graphical representation of the following gas laws:
i.
Boyle’s law
ii.
Charles’ law
Question 2:
a) What instruments are used for measuring pressure of the gas?
b) Convert the following volumes of gas into volumes at s.t.p:
a) 30 cm3 at 261 K and 77 Kpa
b) 450 cm3 at 303 K and 102.7 Kpa
c) 50 cm3 at 620 K and 24.312 x 105 Pa
c) Determine the volume of O2 (at s.t.p) formed when 50.0 g of KClO3 decomposes according to the
following reaction:
2KClO3 (s) → 2KCl (s) + 3O2(g)
Question 3:
a) A sample of gas originally occupies 29.1 L at 0.00C. What is the new volume when it is heated to
150C? (Assume the pressure is constant)
b) Given pressure in units of Pascal, volume in units of cubic metres, temperature in units of Kelvin
and amount of substance in units of moles; derive the units of R, the universal gas constant from
the ideal gas equation.
c) A series of measurements were made in order to determine the molar mass of an unknown gas,
M. Firstly, a flask was evacuated and found to weigh 134.567 g. It was then filled with the gas to
a pressure of 735 torr at 310C and reweighed; its mass was then 137.456 g. Finally, the flask was
filled with water at 310C and found to weigh 1067.9 g. Assuming that the ideal gas equation
applies, calculate the molar mass of the unknown gas, M. (The density of water at 31 oC was
0.997 g/mL)
Question 4:
a) Explain the term ‘partial pressure’.
b) The mass percentage composition of dry air at sea level is approximately: N2: 75.5; O2: 23.2; Ar:
1.3. Calculate the partial pressure of each component when the total pressure is 1.5 atm.
c) A vessel of volume 5dm3 was filled with:
i.
2 mol of H2 (g)
ii.
Then a further 1 mol of Cl2 (g)
iii.
The mixture was sparked leading to the formation of 2 mol of HCl (g)
What is the partial pressure of the component gases at each stage and the total pressure, the
temperature being 250C throughout?
Question 5:
a) Calculate the rate of effusion of methane to that of sulfur dioxide.
b) A certain volume of the gas evolve by photosynthesizing alga culture took 231 s to stream
through a small hole. Under precisely the same conditions, an equal volume of argon took 258 s.
Calculate the molar mass of the unknown gas.
c) Gas X effuses at the rate that is 0.355 times the rate of O2 under the same conditions. Suggest the
name of gas X.
d) The molecular weight of an unknown gas is found by measuring the time required for a known
volume of the gas to effuse through a small pinhole, under constant pressure. The apparatus is
calibrated by measuring the time needed for the same volume of O2 (mol wt = 32.00g/mol) to
effuse through the same pinhole, under the same conditions. The time found for O2 is 60 sec, and
that for the unknown gas is 120 sec. Compute the molecular weight of the unknown gas.
Question 6:
a) Explain the postulates of the kinetic molecular theory of gases.
b) Outline two assumptions made about ideal gases that do not apply to real gases which result in
real gases deviating from the ideal gas behaviour.
c) Show how the ideal gas equation can be modified to account for the behaviour of real gases.
Question 7:
Suppose we have a gas confined to a cylinder with a movable piston. Consider the following changes
(assuming no leaks):
i)
Heat the gas from 298 K to 360 K at constant pressure.
ii) Reduce the volume from 1 L to 0.5 L at constant temperature.
iii) Inject additional gas, keeping temperature and volume constant.
Indicate how each change affects the average distance between molecules, the pressure of the gas,
and the number of moles of gas in the cylinder.
Question 8:
A large evacuated flask initially has a mass of 134.567 g. When the flask is filled with a gas of unknown
molar mass to a pressure of 735 torr at 310C, its mass is 137.456 g. When the flask is evacuated again and
then filled with water at 310C, its mass is 1067.9 g. (The density of water at this temperature is 0.997
g/mL.) Assuming the ideal-gas equation applies, calculate the molar mass of the gas.
Question 9:
An unknown gas composed of homonuclear diatomic molecules effuses at a rate that is 0.355 times the
rate at which O2gas effuses at the same temperature. Calculate the molar mass of the unknown and
identify it.
Question 10:
If 1.000 mol of an ideal gas were confined to 22.41 L at 0.00C, it would exert a pressure of 1.000 atm. Use
the van der Waals equation and Table 10.3 to estimate the pressure exerted by 1.000 mol of Cl2(g) in
22.41 L at 0.00C.