1. Law of Conservation of Matter
Stoichiometry
Mass-Mass Problems

2. What is Stoichiometry?
The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses and volumes of reactants and products involved in a chemical reaction.
Every chemical reaction has it's characteristic proportions.  The method of obtaining these proportions from chemical formulas, equations, atomic weights and molecular weights, and the determination of what and how much is used and produced in chemical processes, is the major emphasis of Stoichiometry.

3. Proportional Relationships
Stoichiometry
mass relationships between substances in a chemical reaction
based on the mole ratio
Mole Ratio
indicated by coefficients in a balanced equation
2 Mg + O2  2 MgO
What is the mole ratio of magnesium metal to magnesium oxide?
What is the mole ratio of magnesium oxide to oxygen?
2 : 2
2 : 1

4. Solving a Stoichiometry Problem
Steps to follow to solve a stoichiometry mass-mass problem
Write out and balance the chemical reaction
Determine what is the given and unknown compounds
Convert given mass to given moles by dividing the molar mass.
Use the molar ratio from the balanced equation (unknown over known) to convert to unknown moles.
Convert unknown moles to unknown mass by multiplying molar mass.
Molar Ratio
Molar Mass of unknown (g)
1 (mol) of known
unknown mol
Given Mass (g)
Molar Mass of known (g)
1 (mol) of unknown
known mol

5. Stoichiometry Problems
When N2 combines with H2 they produce 5.63 g of NH3. How many grams of N2 were needed to produce the ammonia gas?
N H2  2 NH3
4.63g
1.00g
5.63 g
_____?
5.63 g NH3
1 mol NH3
1 mol N2
28.02 g N2 =
17.04 g NH3
2 mol NH3
1 mol N2
(5.63 x 1 x 1 x 28.02) ÷ (17.04 x 2 x 1) =
4.63 g N2
How many grams of H2 are needed for the reaction to take place? Remember the law of conservation of matter.

6. Stoichiometry Problems
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?
4 Al O2  2 Al2O3
6.50 g
5.78g
12.28g
_____?
6.50 g Al
1 mol Al
2 mol Al2O3
g Al2O3 =
26.98 g Al
4 mol Al
1 mol Al2O3
(6.50 x 1 x 2 x ) ÷ (26.98 x 4 x 1) =
12.28 g Al2O3
How much oxygen is needed for the reaction to take place?

7. Stoichiometry Problems
Acetylene gas C2H2 burns in a combustion reaction for welding. How many grams of C2H2 are burned if the reaction produces 75.0 g of CO2?
2 C2H O2  4 CO H2O
22.19g
_____?
75.0 g
75.0 g CO2
1 mol CO2
2 mol C2H2
26.04 g C2H2 =
44.01 g CO2
4 mol CO2
1 mol C2H2
(75.0 x 1 x 2 x 26.04) ÷ (44.01 x 4 x 1) =
22.19 g C2H2