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CDO AP Chemistry Unit 3
Problem Set Part 1 Gases
1. A mixture of hydrogen gas and oxygen gas will remain stable until ignited by a spark. When
the spark initiates the reaction the hydrogen will burn in oxygen to produce water vapor
according to the equation below:
2H2(g) + O2(g)  2H2O (g)
A gas mixture composed of 1.50 moles of H2 and 0.500 moles of O2 and a quantity of inert
He that has a partial pressure of 1.50 atm, is held in a rigid 7.00 L glass vessel at a
temperature of 22.0oC.
a. Calculate the total pressure of the glass vessel.
b. Calculate the mole fraction of each gas in the vessel.
c. A spark is introduced and the reaction above goes to completion. Calculate the molar
composition of the gas mixture at the end of the reaction.
CDO AP Chemistry Christman
Fall 2012
2. In an experiment designed to determine the molar mass of a volatile liquid by the Dumas
method, a 15.0 mL sample of the liquid was gently heated using a water bath in order to fill a
vessel completely with vapor (the excess was allowed to escape from the vessel). The
following data were collected that relate to a 141 mL sample of vapor:
Mass
Temperature
Pressure
0.611 g
98.3oC
755 mmHg
a. Assuming ideal behavior, calculate the molar mass of the volatile liquid?
b. Comment of the effect of each of the following on the calculated molar mass in part
(a) when compared to the ACTUAL molar mass. Explain carefully.
i. The thermometer was incorrectly read as 98.3oC when actually it was 89.3oC.
ii. At the end of the experiment some of the vapor escaped before it was condensed
and massed.
iii. The experiment was repeated, this time using a larger vessel, but still starting
with an amount of liquid that was sufficient to fill the container.
CDO AP Chemistry Christman
Fall 2012
3. Consider three identical flasks (volume is constant) filled with different gases.
Flask X: CO2 at 250 mm Hg and 10oC
Flask Y: N2 at 760 mm Hg and 10oC
Flask Z: H2 at 100 mm Hg and 10oC
a. In which flask (if any) will the molecules have the greatest average kinetic energy?
b. In which flask (if any) will the molecules collide with the wall with the greatest
frequency?
c. In which flask does the gas have the greatest density?
CDO AP Chemistry Christman
Fall 2012
4. Oxygen gas can be produced in small quantities in the laboratory from the decomposition of
potassium chlorate (KClO3):
2KClO3(s)  2KCl(s) + 3O2(g)
If 3.70 g of KClO3 is heated, what volume of O2 gas will be collected over water at 27oC and
735 mmHg? (PH2O at 27oC = 26.7 mmHg)
5. A student collected a sample of hydrogen gas by the displacement of water as shown by the
diagram above. The relevant data are given in the following table.
GAS SAMPLE DATA
Volume of sample
90.0 mL
Temperature
25oC
Atmospheric Pressure
745 mm Hg
Equilibrium Vapor
Pressure of H2O (25oC)
23.8 mm Hg
(a) Calculate the number of moles of hydrogen gas collected.
(b) Calculate the number of molecules of water vapor in the sample of gas.
CDO AP Chemistry Christman
Fall 2012
(c) Calculate the ratio of the average speed of the hydrogen molecules to the average
speed of the water vapor molecules in the sample.
(d) Which of the two gases, H2 or H2O, deviates more from ideal behavior? Explain your
answer.
6. Answer the following questions about carbon monoxide, CO (g), and carbon dioxide, CO2(g).
Assume that both gases exhibit ideal behavior.
(a) Draw the complete Lewis structure (electron dot diagram) for the CO molecule and
for the CO2 molecule.
(b) Identify the shape of the CO2 molecule.
(c) A 1.0 mol sample of CO(g) is heated at constant pressure. On the graph below,
sketch the expected plot of volume verses temperature as the gas is heated.
(d) Samples of CO(g) and CO2(g) are placed in 1 L containers at the conditions in the
diagram below.
CDO AP Chemistry Christman
Fall 2012
(i)
Indicate whether the average kinetic energy of the CO2 is greater than, equal
to, or less than the average kinetic energy of the CO(g) molecules. Justify
your answer.
(ii) Indicate whether the root-mean-square speed of the CO2(g) molecules is
greater than, equal to or less than the root-mean-square speed of the CO(g)
molecules. Justify your answer.
(iii) Indicate whether the number of CO2(g) molecules is greater than, equal, or
less than the number of CO(g) molecules. Justify your answer.
CDO AP Chemistry Christman
Fall 2012
7. For each of the following three statements, write the balanced net ionic equation and answer
the question that follows. There is no need to include the phases. In all cases a reaction
occurs.
a. Aqueous hydrogen peroxide is added to a manganese (IV) oxide catalyst.
i. Describe a simple laboratory test for the gas that is produced.
b. Solid potassium chlorate is heated in a test tube.
i. Describe the change in oxidation state (if any) that the oxygen atoms undergo.
c. Sodium carbonate is heated strongly until no further change occurs.
i. Describe a simple laboratory test for the gas that is produced.
CDO AP Chemistry Christman
Fall 2012