Molecular Composition
of Gases
Stoichiometry of Gases
Objectives
1. Apply the principles of stoichiometry to
determine the volume of any gas that is
produced or consumed in a chemical reaction.
Volume-Volume Calculations
At STP, what volume of hydrogen gas is needed to react
completely with 4.55 L of oxygen gas to produce
water vapor?
2 H2 + O2  2 H2O
 1 mol O2  2 mol H 2  22.4 L H 2 


  9.10 L H 2
(4.55 L O2 )
 22.4 L  1mol O 2  1 mol 
 2 L H2 
  9.10 L H 2
4.55 L O2 
 1 L O2 
Stoichiometry of Gases
2 CO(g) + O2 (g)  2 CO2 (g)
2 molecules CO + 1 molecule O2  2 molecules CO2
2 mol CO + 1 mol O2  2 moles CO2
2 L CO + 1 L O2  2 L CO2
Volume-Volume Calculations
Xenon gas reacts with fluorine gas to produce the gaseous
compound xenon hexafluoride. If a researcher needs
3.14 L of xenon hexafluoride for an experiment, what
volumes of xenon and fluorine should be reacted.
Assume all volumes are measured under the same
conditions of temperature and pressure.
Xe + 3 F2  XeF6
 1 L Xe 
  3.14 L Xe
3.14 L XeF6 
 1 L XeF6 
 3 L F2 
  9.42 L F2
3.14 L XeF6 
 1 L XeF6 
Volume-Volume Calculations
Nitric acid can be produced by the reaction of gaseous
nitrogen dioxide with water.
3 NO2 (g) + H2O(l)  2 HNO3 (l) + NO(g)
If 708 L of NO2 gas react with water, what volume of
NO gas will be produced? Assume the gases are
measured under the same conditions.
 1 L NO 
  236 L NO
708 L NO2 
 3 L NO2 
Volume-Mass Calculations
Aluminum granules are a component of some drain cleaners
because they react with sodium hydroxide to release both
heat and gas bubbles, which help clear the drain clog.
The reaction is…
2 NaOH(aq) + 2 Al(s) + 6 H2O(l)  2 NaAl(OH)4 (aq) + 3H2 (g)
What mass of aluminum would be needed to produce 4.00 L
of hydrogen gas at STP?
 1 mol H 2  2 mol Al  26.981539 g Al 


  3.21 g Al
4.00 L H 2 
1mol Al

 22.4 L H 2  3 mol H 2 
Volume-Mass Calculations
Air bags in cars are inflated by the sudden decomposition
of sodium azide, NaN3, by the following reaction:
2 NaN3 (s)  3 N2 (g) + 2 Na(s)
What volume of N2 gas, measured at 1.30 atm and 87oC,
would be produced by the reaction of 70.0 g of NaN3?
 1 mol NaN 3  3 mol N 2 

  1.62 mol N 2
70.0 g NaN 3 
 65.0099 g NaN 3  2 mol NaN 3 
L  atm  o

1.62 mol ( 0.0821
 87 C  273
nRT
mol  K 

V 

 36.8 L N 2
1.30 atm
P


Volume-Mass Calculations
Rocket propulsion systems and some explosives employ
reactions that suddenly produce a large volume of hot
gas from a small volume of reactants. For example, the
explosive substance, nitroglycerin undergoes the
following decomposition reaction when detonated.
4 C3H5(ONO2)3 (l)  12 CO2 (g) + 10 H2O(g) + 6 N2 (g) + O2(g)
Calculate the total volume of gases produced by the
reaction of 4 moles of nitroglycerin.
(12 mol + 10 mol + 6 mol + 1 mol)(22.4 L/mol) = 650. L