Gas Stoichiometry
Molar Volume of an Ideal Gas
Standard Temperature & Pressure (STP)
Using these with the Ideal Gas Law
Using Stoichiometry and Gas Laws
Calculate the volume of oxygen gas (O2) produced at
1.00 atm and 25oC by the incomplete decomposition of
10.5 g of potassium chlorate: 2KClO3  2KCl + 3O2
– To solve the problem, we use what we know about moles and
the ideal gas law:
1. Convert grams of KClO3 to moles
2. Use a mole ratio to determine moles of O2
3. Calculate the volume of O2 using the ideal gas law
1) 10.5 g KClO3 x 1 mol KClO3 = 0.0856 mol KClO3
122.6 g KClO3
2) 0.0856 mol KClO3 x 3 mol O2 = 0.128 mol O2
2 mol KClO3
3) Use the ideal gas law: PV = nRT
P = 1.00 atm n = 0.128 mol
V=?
T = 25oC  298 K
V = (0.128 x 0.08206 x 298) = 3.13 L
1.00
STP: Standard Temperature and Pressure
• 1 mole of an ideal gas at 0oC (273 K) and 1 atm
has a volume of 22.4 L
V = nRT = (1.00 mol)(0.08206)(273 K) = 22.4 L
P
1.0 atm
• This volume is called the molar volume of an
ideal gas.
1 mol of ANY ideal gas has a volume of 22.4 L at STP:
Equivalence statement: 1.000 mol = 22.4 L (STP)
• STP = the conditions of 0oC and 1 atm
Calculating using STP
A sample of nitrogen gas has a volume of 1.75L at
STP. How many moles of N2 are present?
– We can take a shortcut to the ideal gas law by
using the molar volume of an ideal gas at STP:
1.000 mol = 22.4 L (STP)
SO…
1.75 L x 1.000 mol N2 = 0.0781 mol N2
22.4 L N2
Reactions Involving Gases at STP
Quicklime (CaO) is produced by heating calcium
carbonate (CaCO3). Calculate the volume of CO2
produced at STP from the decomposition of 152 g of
CaCO3 according to the reaction: CaCO3  CaO + CO2
Step 1: Convert the mass of CaCO3 to moles
152 g CaCO3 x 1 mol CaCO3 = 1.52 mol CaCO3
100.1 g CaCO3
Step 3: Use the molar volume of an
ideal gas at STP to determine volume
of CO2
1.52 mol CO2 x 22.4 L CO2 = 34.1 L CO2
Step 2: Determine moles of CO2 using a mole ratio
1 mol CO2
1.0 mol CaCO3 x 1 mol CO2 = 1.52 mol CO2
1 mol CaCO3
Gas Stoichiometry Practice Problems
1.
Calcium oxide can be used to “scrub” carbon dioxide from air:
CaO + CO2  CaCO3
a. What mass of CO2 could be absorbed by 1.25 g of CaO?
b. What volume would this CO2 occupy?
2.
3.
4.
5.
For the reaction 4Al + 3O2  2Al2O3 What volume of oxygen gas
at STP would be needed to react completely with 1.55 g of
aluminum?
An ideal gas has a volume of 50.0 mL at 100oC and a pressure of
690 torr. Calculate the volume of this sample of gas at STP.
A gaseous mixture contains 6.25 g of He and 4.97 g of Ne. What
volume does the mixture occupy at STP?
Using the unbalanced equation: Na + Cl2  NaCl What volume of
chlorine gas, measured at STP, is necessary for the complete
reaction of 4.81 g of sodium metal?