Sunday is
Groundhog
Day!
Enjoy
the
Super
Bowl on
Sunday!
Thursday, Jan. 30th: “A” Day
Friday, Jan. 31st: “B” Day
Agenda
 Homework Questions/Problems/Collect
 Finish Section 12.3: “Molecular Composition of
Gases”
Gay-Lussac’s law of combining volumes, Dalton’s
law of partial pressure, gas stoichiometry 
 Homework:
Practice Pg. 442: #1-3
Section 12.3 review, pg. 442: #1-12
Concept Review: “Molecular Composition of
Gases”
Quiz over section 12.3 next time!
Homework
“Chemistry: Practice
Problems for the
Gas Laws” worksheet
Questions/Problems
Turn In
Gas Reactions Allow Chemical
Formulas to be Deduced
In 1808, Joseph Gay-Lussac made an
important discovery: if the pressure and
temperature are kept constant, gases react in
volume proportions that are small whole
numbers.
Gay-Lussac’s law of combining volumes: The
law that states that the volumes of gases
involved in a chemical change can be
represented by the ratio of small whole
numbers.
Gas Reactions Allow Chemical
Formulas to be Deduced
Hydrogen molecules combine with chlorine
molecules in a 1:1 volume ratio to produce
twice the volume of hydrogen chloride, HCl.
Dalton’s Law of Partial Pressure
In 1805, John Dalton showed that in a mixture of
gases, each gas exerts a certain pressure as if it
were alone with no other gases mixed with it.
Partial pressure: the pressure of each gas in a
mixture.
Dalton’s law of partial pressure: The law that
states that the total pressure of a mixture of
gases is the sum of the partial pressures of the
component gases.
Dalton’s Law of Partial Pressure
Ptotal = PA + PB + PC
Ptotal is the total pressure, and PA, PB, and PC
are the partial pressures of each gas.
Dalton’s Law of Partial Pressure
Dalton’s law of partial pressure is explained by
the kinetic molecular theory.
All the gas molecules are moving randomly,
and each has an equal chance to collide with
the container wall.
Each gas exerts a pressure proportional to its
number of molecules in the container. The
presence of other gas molecules does not
change this fact.
Gas Stoichiometry
Ratios of gas volumes will be the same as mole
ratios of gases in balanced equations.
Avogadro’s law shows that the mole ratio of two
gases at the same temperature and pressure is the
same as the volume ratio of the two gases.
This greatly simplifies the calculation of the
volume of products or reactants in a chemical
reaction involving gases.
Gas Stoichiometry
For example, consider the Haber process for the
production of ammonia gas…
3 H2(g) + N2(g) → 2 NH3(g)
3 L of H2 react with 1 L of N2 to form 2 L of NH3,
and no H2 or N2 is left over.
If we know the number of moles of a gaseous
substance, we can use the ideal gas law to
calculate the volume of that gas.
And that’s what we’re going to do………..
Sample Problem G, pg. 441
How many liters of hydrogen gas will be produced at
280.0 K and 96.0 kPa if 1.74 mol of sodium react
with excess water according to the following
equation?
2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g)
1.74 mol
L=?
Use stoichiometry: change mol Na mol H2
Use mol H2 in the ideal gas law: PV = nRT
(96.0 kPa) V = (.870 mol) (8.314) (280.0 K)
V = 21.1 L H2
Additional Practice
A student wishes to prepare oxygen by using the
thermal decomposition of potassium chlorate,
KClO3. Given that the gas will have a temperature
of 700˚C and a pressure of 98.6 kPa, how much
potassium chlorate will be necessary to produce
125 mL of oxygen?
2 KClO3
3 O2 + 2 KCl
Use PV=nRT to find moles of O2
(98.6 kPa) (.125 L) = n(8.314)(973 K) = .0015 mol O2
Use stoichiometry to find moles of KClO3.
Change mol of KClO3 to g of KClO3. = .122 g KClO3
Additional Practice #2
Methane has a volume of 0.65 L when under
100 kPa of pressure at a temperature of 305 K.
Using the balanced equation below, find the mass
of oxygen that is needed to use up all of the
methane.
CH4 + 2 O2 CO2 + 2 H2O
Use PV = nRT to find moles of methane
(100 kPa) (0.65 L) = n (8.314) (305 K)
n = .026 mol methane
Use stoichiometry and molar mass to find mass of
oxygen needed
1.66 g O2
Homework
Practice, pg. 442: #1-3
Section 12.3 review, pg. 442: #1-12
Concept Review: “Molecular Composition of
Gases”
Next Time:
12.3 Quiz
Lab Write-up
Mini Lab!