Ch. 11: Molecular Composition
of Gases
11.1 VOLUME-MASS RELATIONSHIPS OF
GASES
Gay-Lussac’s Law of Combining Volumes of
Gases
 at constant T and P, coefficients in balanced
equation represent ratio of volumes of gaseous
reactants too
2H2(g) + O2(g)  2H2O(g)
2L 1L
2L
Example
3O2(g)  2O3(g)
 How many liters of O3 can be made from 12 L of
O2?
2 L O3
12 L O 2 
 8.0 L O 3
3 L O2
 How many moles of O2 are needed to make 24
moles of O3?
3 mol O 2
24 mol O3 
 36 mol O 2
2 mol O3
 How many molecules of O3 can be made from 18
molecules of O2?
2 molecules O3
18 molecules O 2 
 12 molecules O3
3 molecules O 2
Avogadro’s Law
 equal volumes of gases at the same T and P contain
equal numbers of molecules
 at same T and P, volumes varies directly with
number of moles (n)
 V = kn
Molar Volume of Gases
 like molar mass
 mass of one mole of substance
 but with volume
 volume of one mole of substance
 because of Avogadro’s law, one mole of any gas has
the same volume as any other gas at the same T and P
Molar Volume of Gases
 Standard Molar Volume of Gas
 volume of one mole of gas at 1 atm and 0°C is 22.4L
 22.4 L of any gas has one mole of particles but has
different masses
 Standard Temperature and Pressure


STP
1 atm and 0°C
Molar Volume of Gases
Example
 A chemical reaction produces 0.0680 mol of
oxygen gas. What volume in liters is occupied by
this gas sample at STP?
1
mol : 22.4 L
22.4 L
0.0680mol 
 1.5232  1.52 L
1mol
Example 2
 A chemical reaction produced 98.0 mL of sulfur
dioxide gas at STP. What was the mass of gas
made?



convert mL to L
convert L to moles using molar volume
convert moles to grams using molar mass
1L
1mol 64.065 g
98.0mL 


 0.280 g
1000mL 22.4 L
1mol
Example 3
 A chemical reaction produced 3.1 g of CO2 gas.
What volume will it have in mL at STP?



convert grams to moles
convert moles to liters
convert liters to milliliters
3.1gCO2 
1molCO2
22.4 LCO2 1000mLCO2


 1600mLCO2
44.0098 gCO2 1molCO2
1LCO2
Example 4
 How many moles of gas are in a container with a
volume of 2.46 L at STP?
1mol
2.46 L 
 0.110
22.4 L