Take Home (Practice) Test Ch10
1) PV, TV, nV, TP are named gas relationships. Fill in a graph of how each gaseous
quantity will change vs. the other on the sample axes below. Assume the quantities not
named on the graph axes are constant. Assume all gases are ideal.
Pressure (atm)
Pressure (atm)
Volume (mL)
Temperature (K)
Volume (mL)
Volume (mL)
Temperature (K)
Moles of gas (mol)
2) A 10.0L cylinder contains 32.0g of O2(g) and 14.0g of N2(g).
a) calculate the total pressure, in atm, of the gaseous mixture at 307K.
b) the temperature of the gas mixture in the cylinder is decreased to 270K.
Calculate each of the following:
i) the mole fraction of O2(g) in the cylinder.
ii) the partial pressure, in atm, of O2(g) in the cylinder.
c) If the cylinder develops a pinhole-sized leak and some of the gaseous mixture
escapes, would the ratio:
moles of N2(g)
moles of O2(g)
increase, decrease or remain the same? Justify your answer.
3) A gaseous hydrocarbon (a substance containing ONLY carbon and hydrogen (thus the
name, “hydrocarbon”)) has the empirical formula C2H2.
a) The gaseous hydrocarbon’s density at 373K and 2.0atm is 3.398g⋅L-1. Determine the
molar mass of the hydrocarbon.
b) Determine the molecular formula of the hydrocarbon.
4) Consider the hydrocarbon ethane, C2H6 (molar mass = 30 g·mol-1)
a) write the balanced chemical equation for the combustion of ethane to yield carbon
dioxide and water.
b) What volume of dry carbon dioxide, measured at 20°C and 685mmHg, will result from
the combustion of 5.50g of ethane?
c) The complete combustion of 20.0g of ethane releases 452 kJ of heat. On the basis of
this information, calculate the value of ΔH for the complete combustion of one mole of
ethane.
d) Under identical conditions, a sample of an unknown gas effuses into a vacuum at half
the rate that a sample of ethane effuses. Calculate the molar mass of the unknown gas.
Include units on your answer.
5) A rigid 5.50L flask contains a mixture of 1.50 moles of H2(g), 0.85 mole of N2(g), and
sufficient Ar(g) so that the partial pressure of Ar(g) in the flask is 1.00 atm. The
temperature is 295.0C.
a) calculate the total pressure in the flask.
b) Calculate the mole fraction of H2(g) in the flask.
c) Calculate the density (in g·L-1) of the mixture in the flask.
d) The mixture in the flask is ignited by a spark, and the reaction represented below
occurs until one of the reactants is entirely consumed.
3H2(g) + N2(g)  2NH3(g)
Give the mole fraction of all species present in the flask at the end of the reaction.
6) A student performs an experiment to determine the molar mass of an unknown gas. A small amount of
the pure gas is released from a pressurized container and collected in a graduated tube over water at room
temperature, as shown in the diagram above. The collection tube containing the gas is allowed to stand for
several minutes, and its depth is adjusted until the water levels inside and outside the tube are the same.
Assume that:
• the gas is not appreciably soluble in water
• the gas collected in the graduated tube and the water are in thermal equilibrium
• a barometer, a thermometer, an analytical balance, and a table of the equilibrium vapor pressure of
water a various temperatures are also available.
a)
b)
c)
d)
write the equation(s) needed to calculate the molar mass of the gas.
List the measurements that must be made in order to calculate the molar mass of the gas.
Explain the purpose of equalizing the water levels inside and outside the gas collection tube.
The student determines the molar mass of the gas to be 24.0 g⋅mol-1. Determine the percent error
in the experimental value, assuming the unknown gas is methane (molar mass 16 g⋅mol-1). Work
required.
e) Explain why the student must use information from the table of the equilibrium vapor pressures of
water during calculations of the molar mass.
f) Calculate the number of moles of water vapor in the sample of gas.
g) Calculate the mole fraction of ethane in the “wet” sample of gas.
h) Calculate the average speed of ethane gas molecules at 25.0˚C.
i) Why does water vapor act less like an “ideal gas” than ethane at 25˚C?
j) Draw the Lewis dot structure for ethane and identify the hybridization of each carbon atom.
7) Two flasks are connected by a stopcock as shown below. The 10.0L flask contains
He(g) at a pressure of 2.0 atm, and the 5.0L flask contains Ar(g) at a pressure of 0.50L.
Calculate the total pressure of the system after the stopcock is opened. Assume that the
temperature remains constant.
8) Butane, C4H10(l), has density of 0.504 g·mL-1 at 10C. A 115mL sample of C4H10(l)
measured at 20°C reacts completely with excess oxygen as represented by the equation
below:
2C4H10(l) + 13O2(g)  8CO2(g) + 10H2O(l)
Calculate the total number of moles of gaseous products formed.
9) Samples of CH4(g) and C2H6(g) are placed in 1.0L containers at the conditions indicated
in the diagram below.
i) Indicate whether the average kinetic energy of the CH4(g) molecules is greater
than, equal to, or less than the average kinetic energy of the C2H6(g) molecules.
Justify your answer.
ii) Indicate whether the root-mean-square speed of the CH4(g) molecules is greater
than, equal to, or less than the root-mean-square speed of the C2H6(g)
molecules. Justify your answer.
iii) Indicate whether the number of the CH4(g) molecules is greater than, equal to, or
less than the number of C2H6(g) molecules. Justify your answer.
iv) Indicate which gas will diffuse faster. Justify your answer.
v) Indicate which gas would deviate the most from an ideal gas. Justify your answer.
10) Consider two containers of volume 1.0L at 298K, as shown above. One container
holds 0.250 mol F2(g) and the other holds 0.250 mol Cl2(g) . The average kinetic energy of
the Cl2(g) molecules is 4.5x10-20J. Assume that both gases exhibit ideal behavior.
a) Is the pressure holding the F2(g) less than, greater than or equal to the pressure in
the container holding the Cl2(g)? Justify your answer.
b) What is the average kinetic energy of the F2(g) molecules?
c) The molecules of which gas, F2(g) or Cl2(g), have the greater average speed?
Justify your answer.
d) What change could be made that would increase the average kinetic energy of the
molecules in the container?
e) If the volume of the container holding the Cl2(g) was decreased to 0.250L at 298K,
what would be the change in each of the following variables. In each case, justify
your answer.
i. The pressure within the container
ii. The average speed of the Cl2(g) molecules