1. Ch. 11: Molecular Composition of Gases
11.3: Stoichiometry of Gases

2. Stoichiometry of Gases
Remember that coefficients give you
ratio of moles
ratio of molecules
ratio of volumes
even if the gases have different conditions
only when gases are all at same T and P

3. Calculations Review getting moles from grams?
molar mass (periodic table)
getting volume from moles for gas?
at STP only
Molar Volume (22.4 L = 1 mol)
any conditions
Ideal Gas Law (PV = nRT, solve for V)
getting from one type gas to new gas
mole ratio (coefficients in equation)

4. CaCO3(s)  CaO(s) + CO2(g)
Example 1
Calcium carbonate can be heated to make calcium oxide according to the following equation:
CaCO3(s)  CaO(s) + CO2(g)
How many grams of CaCO3 must be decomposed to make 5.00 L of CO2 at STP?
volume of CO2  moles CO2 (molar volume)
moles CO2  moles CaCO3 (mole ratio)
moles CaCO3  grams CaCO3 (molar mass)

5. Example 1 find moles of CO2 convert mol CO2 to mol CaCO3
convert mol CaCO3 to grams

6. WO3(s) + 3H2(g)  W(s) + 3H2O(l)
Example 2
Tungsten is made industrially by:
WO3(s) + 3H2(g)  W(s) + 3H2O(l)
How many liters of hydrogen gas at 35°C and atm are needed to react completely with 875 g WO3?
grams of WO3  moles WO3 (molar mass)
moles WO3  moles H2 (mole ratio)
moles H2  volume H2 (PV = nRT)

7. Example 2
find moles of WO3
find moles of H2
find volume of H2

8. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Example 3
Combustion of methane gas produces carbon dioxide gas and water vapor.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
If the reaction produces 213 L of CO2 at STP, what volume of O2 must have been used at 23C and 0.89 atm?
volume of CO2  moles CO2 (molar volume)
moles CO2  moles O2 (mole ratio)
moles O2  volume O2 (PV = nRT)

9. Example 3 find moles of CO2 convert mol CO2 to mol O2
Find volume of O2