Unit 1 Gases
Ideal Gases
 Objectives
Compute the value of an unknown using the ideal
gas law.
2. Compare and contrast real and ideal gases.
1.
Avogadro’s Principle and Molar Volume
 Avogadro’s principle states that equal volume of all
gases, measured under the same conditions of
pressure and temperature, contain the same number
of particles.
 At STP, the volume of one mole of gas is 22.4 L(this is
called the molar volume)
 1dm3 = 1 L; 1L= 1000mL ; 1mL= 1cm3
 From last year, to convert between grams and moles of
a substance we used its molar mass.
Avogadro’s Law
Converting between moles and grams:
How many moles are 98.32g CO2?
Calculate molar mass CO2 (use periodic table)
Molar mass= 12 + (2 x 16) =44.0 g/1 mol
To convert grams to moles:
98.32g x 1 mol = 2.23 mol CO2
44g
2. How many grams of carbon dioxide, CO2, will occupy
a volume of 500.0 mL at STP?
V= 500.0mL =0.5 L
Molar mass CO2= 44g/mol
At STP, molar volume is : 22.4L/1 mol
Classwork: p 132 # 1 (a,b), 2(b,c), 3 (b,c)
The ideal gas law
 Considers that amount of gas varies.
 New variable
 n: number of moles of gas (mol)
 Ideal gas constant (R)
 R= 8.314 L kPa
(when pressure is measured in kPa)
mol K
 R= 0.0821 L atm (when pressure is measured in atm)
mol K
 Ideal gas law:
PV= nRT (T must be in Kelvin)
Using the ideal gas law
1. A deep underground cavern contains 2.24x106 L of
methane gas (CH4) at a pressure of 1500 kPa and a
temperature of 315K. How many moles of CH4 does
the cavern contain?
Using the ideal gas law
2. How many moles of oxygen will occupy a volume of
2.5 L at 1.2 atm and 25C? (0.123 moles)
Classwork: p 133 # 7,8 and p141 # 3,4
Ideal gas variations
 For calculating molar mass
 M= mRT
PV
M: molar mass (g/mol)
m: mass (g)
 For calculating density
 D= MP
D: density (g/L)
RT
 Sample problem
1.
What is the molar mass of sulfur dioxide, SO2, if 300
mL of the gas has a mass of 0.855 g at STP?
2. At what temperature will 5.00g of Cl2 exert a
pressure of 115 kPa at a volume of 750 mL?
3. If the density of a gas is 1.2 g/L at 0.920 atm and
20C, what is its molar mass?
Classwork: p 133 #5,6, 9 and p141 #2,5