7.2 STOICHIOMETRY – MASS RELATIONSHIPS IN CHEMICAL EQUATIONS
Previous Knowledge:
The coefficients in a balanced chemical equation represent the mole ratio of the chemicals
in the reaction. This is a relative amount required to produce a certain amount expected.
Stoichiometry is the study of the relationship between the amounts of reactants and products in a
chemical reaction.
A stoichiometric amount is an amount of reactants that is in the same proportion as the reactant
coefficients in the balanced chemical equation.
Consider the following reaction:
SO2(g) + CaCO3(s)  CaSO3(s) + CO2(g)
Figure 1: Ca(CO3)2 is used
to scrub SO2 from toxic
factory emissions.
The ratio of sulfur dioxide to
calcium carbonate is a 1:1
ratio. This means that sulfur
dioxide
and
calcium
carbonate
are
in
stoichiometric amounts in
any mixture where the
number of moles of each
compound is the same. If
there
is
less
calcium
carbonate than in the ratio,
unused sulfur dioxide will
escape and interact with
water molecules to produce
acid rain (in the form of
sulfuric acid).
Solving Stoichiometric Problems
We are able to answer these types of equations because of our balanced equation. If we know the
amount of one substance in a chemical reaction (particles, moles, mass), you can calculate the
amount of any other substance in the reaction (particles, moles, mass) based on this balanced
equation.
General Process for Solving Stoichiometry Problems:
Step 1: Write a balanced chemical equation.
Step 2: If you are given mass or number of particles (molecules, formula units,
compounds, etc.), convert it to moles
Step 3: Calculate the number of moles of the required substance based on the number
of moles of the given substance with the mole ratio
Step 4: Convert the moles of the required substance to mass of number of particles
(molecules, formula units, compounds, etc.)
Example Problem 1:
Hydrazine, N2H4 and dinitrogen tetroxide, N2O4 was used to launch a lunar module. These
two compounds react to form nitrogen gas and water vapour. If 50.0 g of hydrazine reacts
with sufficient dinitrogen tetroxide, what mass of nitrogen gas is formed?
1)
Balanced Equation: 2 N2H4(l) + N2O4(l)  3 N2(g) + 4 H2O(g)
2)
Mass  Moles
3) Compare using Mole Ratio
4) Moles  Mass
Example 2:
A typical astronaut exhales 8.8 x 102 g of carbon dioxide daily. Determining the mass of
lithium hydroxide required to react with this mass of CO2.
Homework: Page 321-324 Practice Page 323 #1-3
Questions Page 325 #2,4,6,8,10 (Even Problems)