What happens when we change the quantity of
gas? When using the combined gas law, n is
constant.
Law of Combining Volumes
 See Fig. 1 on p. 576
 Which test tube contains H2(g)?
2 H2O(g)  2 H2(g) + O2 (g)
Gay-Lussac’s Law of Combining Volumes: when gases react, the
volumes of the reactants and products, measured at equal
temperatures and pressures, are always in whole number ratios
(whole number ratios are the stoichiometric coefficients)
How can 2 samples of gas that contain particles of
different mass have equal pressures at the same T?
Avogadro’s Hypothesis: equal volumes of all ideal
gases at the same T and P contain the same number
of molecules Fig 4 p. 578
Molar Volume
 At STP, 1 mole of an ideal gas will have a volume of ______.
 At SATP, 1 mole of an ideal gas will have a volume of 24.8 L.
 Then how do gases differ from one another???
 mass (due to different molar masses)
 density
Ideal Gas Law
R= universal gas constant
R= 8.314 kPa L/mol K
Derive R @ STP…
R= 0.082058 atm L/ mol K
R= 62.37 mmHg L/ mol K
Dalton’s Law of Partial
Pressures
Ptotal= P1 + P2 + P3 + … +Pn
Imagine mixing 3 different gases
each having a different
pressure…
What would the final pressure
be?
Dalton’s Law of Partial
Pressures: the total pressure
of a mixture of gases is the
sum of the pressures of each
of the individual gases
Ex. What is the pressure of O2 in
the atmosphere?
Practice
Pg. 594 #1-4
Pg. 596 #1-3