Chapter 12
Stoichiometry
Goal
 Calculate quantities of reactants and
products needed in chemical rxns using
balanced chemical equations.
Using an Equation as a Recipe
 Ingredients: Frame, Seat, Wheels,
Handlebar, Pedals
 Balanced equation:
1F + 1S + 2W + 1H + 2P
1 FSW2HP2
How many pedals are needed to
make 128 bicycles?
128 FSW2HP2
2P
= 256 pedals
1 FSW2HP2
1F + 1S + 2W + 1H + 2P
1 FSW2HP2
Examining a Balanced
Chemical Equation
2H2S(g) + 3O2(g)
2SO2(g) + 2H2O(g)
 2 moles of H2S reacts with 3 moles of O2 to
produce 2 moles of SO2 and 2 moles of
H2O.
 4 moles of H2S reacts with ___moles
of O2
6
 10.5
____ moles of O2 are needed to produce 7
moles of H2O
 How to solve these is on the next slide.
Mole-Mole Ratio Problems
4 mol H2S 3 mole O2
2 mol H2S
= 6 mol O2
7mol H2O 3 mole O2
2 mol H2O
= 10.5 mol O2
The mol-mol ratio comes from the coefficients in
the balanced equation. In the balance equation, O2
has a coefficient of 3, H2O has a coefficient of 2,
and H2S has a coefficient of 2.
Mass-Mass Calculation
 Write the balanced chemical equation
 Label above the equation what you are
given and what you are trying to find.
 Convert grams of your given to moles using
molar mass
 Multiply by the mole-mole ratio from the
balanced chemical equation
 Convert moles of unknown to grams
Bringing It All Together
 How many grams of ammonia (NH3 )
should be produced when 6.00g nitrogen
react with excess hydrogen?
 Solve: Write the balanced equation with
the information labeled above it.
6.00 g
excess
?g
N2 + 3H2
2 NH3
Bringing It All Together
 Convert given to moles:
6.00g N2 1 mole N2
28 g N2
= 0.214 mol N2
 Multiply by the mole ratio:
0.214 mol N2
2 mol NH3
1 mol N2
= 0.428 mol NH3
Wrapping It Up
Convert moles of unknown to grams:
0.428 mol NH3
17g NH3
= 7.28g NH3
1 mol NH3
Summary:
When 6.00g of nitrogen(N2) reacts with an excess of
Hydrogen (H2), 7.28g of ammonia (NH3)
should be produced.
Now You Try!
What mass of aluminum is required to
completely react with 7.80g oxygen in a
synthesis reaction?
Write the balanced equation
4Al + 3O2
2Al2O3
Label your given and unknown
information
Since you were not
given info about
this compound,
don’t worry about it
and don’t use it.
7.80 g
4Al + 3O2
2Al2O3
NEXT?...
 Convert given to moles:
7.80g O2
1 mol O2
= 0.244mol O2
32g O2
 Multiply by the mole ratio:
0.244mol O2
4 mol Al
3 mol O2
= 0.325 mol Al
AND FINALLY!!
Convert moles of unknown to grams:
0.325 mol Al
27g Al
1 mol Al
= 8.78g Al
8.78g of aluminum (Al) should be required to react
completely with7.80g of oxygen (O2) in the synthesis
of aluminum oxide (Al2O3).
Mass-Volume Calculation
 Write the balanced chemical equation
 Label above the equation what you are given and
what you are trying to find.
 Convert grams of your given to moles using molar
mass
 Multiply by the mole-mole ratio from the balanced
chemical equation
 Convert moles of unknown to liters (a.k.a dm3)
using molar volume (22.4L/ 1mol)
Bringing It All Together
 How many liters of oxygen are necessary
for the combustion of 134g of magnesium,
assuming the reactions occurs at STP?
 Solve: Write the balanced equation with
the information labeled above it.
134 g
?L
2Mg + O2
2 MgO
Bringing It All Together
 Convert given to moles:
134g Mg 1 mole Mg
= 5.514 mol Mg
24.3 g Mg
 Multiply by the mole ratio:
5.514 mol Mg
1 mol O2
2 mol Mg
= 2.757 mol O2
Wrapping It Up
Convert moles of unknown to liters:
2.757 molO2
22.4 LO2
= 61.8 L O2
1 mol O2
61.8 L of O2 are required to completely combust
134g Mg in the synthesis of MgO.
Volume-Mass Calculation
 Write the balanced chemical equation
 Label above the equation what you are
given and what you are trying to find.
 Convert liters of your given to moles using
molar volume
 Multiply by the mole-mole ratio from the
balanced chemical equation
 Convert moles of unknown to grams using
molar mass
Bringing It All Together
 Find the mass of sulfur (S8) required to react
with oxygen to produce 2.47L of sulfur
dioxide gas at STP.
 Solve: Write the balanced equation with
the information labeled above it.
?g
2.47L
S8 + 8 O2
8 SO2
Bringing It All Together
 Convert given to moles:
2.47L SO2 1 mole SO2
22.4 L SO2
= 0.1103 mol SO2
 Multiply by the mole ratio:
0.1103 mol SO2
1 mol S8
8 mol SO2
= 0.01378 mol S8
Wrapping It Up
Convert moles of unknown to grams:
0.01378 mol S8
256g S8
= 3.53 g S8
1 mol S8
3.53g of sulfur (S8) are required to produce
2.47 L of sulfur dioxide (SO2).
Volume-Volume
Calculations
 Write the balanced chemical equation
 Label above the equation what you are
given and what you are trying to find.
 Convert liters of your given to moles using
molar volume (22.4L/1mol)
 Multiply by the mole-mole ratio from the
balanced chemical equation
 Convert moles of unknown to liters using
molar volume (22.4L/1 mol)
Bringing It All Together
 What volume of H2S gas is needed to
produce 14.2 L of water at STP?
 Solve: Write the balanced equation with
the information labeled above it.
?L
14.2L
2H2S + 3O2
2 SO2 + 2 H2O
Bringing It All Together
 Convert given to moles:
14.2 L H2O 1 mole H2O
22.4 L H2O
= 0.6339 mol H2O
 Multiply by the mole ratio:
0.6339 mol H2O
2 mol H2S
2mol H2O
= 0.6339 mol H2S
Wrapping It Up
Convert moles of unknown to liters:
0.6993 mol H2S
22.4 LH2S
= 14.2 L H2S
1 mol H2S
14.2 L of H2S are required to produce 14.2 L H2O.