Stoichiometry
CHAPTER 10
 Stoichiometry is the mass and amount relationship
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of reactant and products.
Consider the following reaction ;
4NH3(g) + 5O2(g)--- 4NO(g) +6H2O(g)
The equation states that 4 mol ammonia react with 5
mol oxygen to produce 4 mol NO and 6 mol H2O.
Suppose you have 2.4 mol of NH3. How many mol
O2 are needed?
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 First you have to carry a mole to mole conversion.
 mol NH3-mol O2
 2.4 mol of ammonia х 5 mol of oxygen/4 mol of
ammonia
 =3.0 mol of oxygen.
Class Practice
 You are given 450 g of iso amyl alcohol(C5H11OH).
Convert to moles of iso amyl alcohol?
Mass to mass problem solving plan
 1. Always begin with a balanced chemical equation.
 2. Use the molar mass of the given substance to
convert from mass of given to product of given.
 3. Use the mole ratio of coefficients from the
balanced chemical reaction to change from amount
of given to amount of unknown.
 4. Use the molar mass of unknown to convert from
amount of unknown to mass of unknown.
Mole to mass problem solving plan
 Always begin with a balanced chemical equation.
 Use the mole ratio of coefficients from the balanced
chemical reaction to change from amount of given to
amount of unknown.
 Use the molar mass of unknown to convert from
amount of unknown to mass of unknown.
Homework
 Page 356
 4 and 5
Limiting reactant, excess reactant
 Limiting Reactant - The reactant in a chemical
reaction that limits the amount of product that can
be formed. The reaction will stop when all of the
limiting reactant is consumed.
 Excess Reactant - The reactant in a chemical
reaction that remains when a reaction stops when
the limiting reactant is completely consumed. The
excess reactant remains because there is nothing
with which it can react.
Limiting and Excess Reagents
Class Practice
 Carbon monoxide can be combined with hydrogen to
produce methanol, CH3OH. If you had 152.5 g CO
and 24.50 g H2. What mass of CH3OH could be
produced?
Home work
 Page 360
 1 and 2
Yields
 The mass of product expected from stochiometric
calculations is called the theoretical yield.
 Actual yield is the mass of the product actually
obtained. This is usually less than the actual yield.
 The ratio of actual yield to the theoretical yield
multiplied by 100 is the percentage yield.
Class Practice
 A student is synthesizing aspirin by adding 200.0 g
of salicylic acid to an excess of acetic anhydride.
Calculate the percentage yield if 231 g of aspirin is
produced.
Home work
 Page 366
 4,6,7
Application of stochiometry
 Stochiometry has a number of applications, from
banana flavoring, to cosmetics to aspirin.
 One important application of stochiometry is in air
bags used in cars.
 The function of the airbags is to protect the occupant
from injuring themselves by hitting against the
windshield or steering wheel or the instrument
panel. Stochiometry is used to make sure the airbags
do not overinflate or underinflate. Under inflation
would not protect the occupant and overinflation
may cause the bag to rupture making them useless.
 The ignitor provides heat energy to start a reaction in
a mixture called the gas generant. The gas generant
is a solid mixture of sodium azide (NaN3), and an
oxidizer. The gas that inflates the bag is pure
nitrogen(N2) which is produced by the following
reaction.
 2NaN3(s)2Na(s) +3N2(g)
 this reaction alone cannot inflate the gas fast
enough and also sodium metal is very reactive; so
ferric oxide (Fe 2O3) which is also added to the
generant reacts with sodium metal.
 6Na(s) + Fe2O3(s)3Na2O(s) + 2Fe(s) + 418kJ
 This exothermic reaction raises the temperature more
than a hundred degrees so that the gas fills the bag
faster.
 The Na2O is unsafe as it is an extremely corrosive
substance. Eventually it reacts with CO2 and moisture
from the air to form sodium bicarbonate or baking soda.
 To calculate the amount of gas generant necessary air
bag designer must know the stochiometry of reactions
and account for energy changes in the reactions, which
may change the temperature and thus the density of the
gas.
Class Practice
 Assume that 65.1 L of N₂ gas is needed to inflate an
air bag to the proper size. What mass of NaN₃ must
be included in the gas generant to generate this
volume of N₂?
Home work
 Page 378
 15, 16,17