Download Unit 4 Section A.17-18

Unit 4
Section A.17-18
In which you will learn about:
•Molar volume
•Avogadro’s law
•Stoichiometry with gases
A.17 Ideal Gases and Molar Volume
• Thus far, you have considered gas behavior under
differing conditions of volume, pressure, and
temperature.
– One variable that you have not fully considered is the
actual amount of the gas sample; that is, the number
of gas molecules contained in a particular sample.
– If you have the same volumes of oxygen gas, nitrogen
gas, and carbon dioxide gas in three different balloons
at the same temperature and pressure, how do the
numbers of molecules compare?
Avogadro’s Law
• That same question was investigated in the
early 1800s by the Italian lawyer and
mathematical physics professor Amedeo
Avogadro
– By making careful observations of gas samples like
those you have worked with, he proposed:
• Equal volumes of all gases at the same temperature and
pressure contain the same number of molecules =
Avogadro’s Law
Molar Volume
• One useful consequence of Avogadro’s law is
that all gases have equal molar volumes if
they are measured at the same temperature
and pressure.
– Molar volume = volume occupied by one mole of
a substance
– @ STP, molar volume = 22.4 L/mol
– There is no similar simple relationship between
moles of various solids or liquids and their
corresponding volumes
Gas Stoichiometry
• The realization that all gases have the same molar
volume under the same conditions greatly
simplifies our thinking about chemical rxns
involving gases
– For example, consider the following chemical
equations that involve gaseous reactions:
N2 (g) + O2 (g)  2 NO (g)
2 H2 (g) + O2 (g)  2 H2O (g)
– You have learned that coefficients in chemical
equations indicate the relative numbers of molecules
or moles of reactants and products
– Based on Avogadro’s law, similar calculations can also
involve volumes of gaseous reactants and products.
Sample Problem 1
• What volume (in liters) of NO (g) is produced as 3
L N2 (g) and 3 L O2 (g) react? See equation on
previous slide.
• Because the amounts of both the reactants have
tripled, the amount of product will also be
tripled: 6 L NO (g) will be produced.
• Although it works in this problem, note that,
unlike mass, volumes are not necessarily
conserved in a chemical reaction:
– 100 mL + 200 mL COULD yield 200 mL , not 300 mL
Sample Problem 2
• In the following equation, what volume of C2H6
(g) is needed to produce 12 L CO2 (g)? All
measurements are at the same temperature.
2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O (g)
• From the chemical equation, we know that 2 L
C2H6 will produce 4 L CO2. If we want to produce
12 L of CO2, which is three times more than is
represented in the equation, the we must start
with three times as much C2H6 = 6 L C2H6.
Sample Problem 3
• Using the same scenario as sample problem 2,
how many liters of O2 would be needed to
produce 12 L CO2?
• 12 L CO2 x (7 L O2/ 4 L CO2) = 21 L O2
Mole Ratio!
A.18 Molar Volume and Reactions of
Gases (HOMEWORK)
1) What volume would be occupied by 3 mol CO2
(g) at 0°C and 1 atm?
2) In a certain gaseous reaction, 2 mol NO react
with 2 mol O2:
2 NO (g) + O2 (g)  2 NO2 (g)
a) Given the same conditions of temperature and
pressure, what volume of O2 (g) would react with 4
L NO gas?
b) How might chemists use their knowledge of molar
volumes to monitor the progress of this reaction?
Homework Cont’d
3) Toxic carbon monoxide (CO) gas is produced when
fossil fuels, such as gasoline, burn without sufficient
oxygen gas. The CO can eventually be converted to CO2
in the atmosphere. Automobile catalytic converters are
designed to speed up this conversion:
Carbon monoxide gas + oxygen gas  carbon dioxide gas
a) Write the balanced equation for this conversion.
b) How many moles of oxygen gas would be needed to
convert 50.0 mol carbon monoxide to carbon dioxide?
c) What volume of oxygen gas would be needed to react
with 968 L CO? (Assume both gases are at the same
temperature and pressure).