Transcript mole ratio - wbm

Stoichiometry
Chapter 9
Page 1
Stoichiometry
• Quantitative relationship between two
substances
• Composition stoichiometry: mass
relationships of elements in compounds
• Reaction stoichiometry: mass
relationships between reactants and
products
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Mole ratio
• Used in all reaction stoichiometry
problems
• Gets us from given to unknown
• Conversion factor
– Relates amounts in moles for any two
substances in a reaction
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• CH4(g) + 2O2(g)  2H2O(g) + CO2(g)
1 mol CH 4 (g)
2 mol O 2 (g)
1 mol CH 4 (g)
2 mol H 2 O(g)
1 mol CH 4 (g)
1 mol CO 2 (g)
2 mol O 2 (g)
1 mol CH 4 (g)
2 mol H 2 O(g)
1 mol CH 4 (g)
1 mol CO 2 (g)
1 mol CH 4 (g)
2 mol O 2 (g)
2 mol H 2 O(g)
2 mol O 2 (g)
1 mol CO 2 (g)
2 mol H 2 O(g)
1 mol CO 2 (g)
2 mol H 2 O(g)
2 mol O 2 (g)
1 mol CO 2 (g)
2 mol O 2 (g)
1 mol CO 2 (g)
2 mol H 2 O(g)
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Mole ratios
• Are exact
• Do not limit the number of significant
figures
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Molar mass
• Used in many stoichiometry problems
• Mass in grams of one mole of a
substance
1 mol H 2 O
18.02 g H 2 O
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18.02 g H 2 O
1 mol H 2 O
Moles to moles
• Amount of given substance in moles
• To
• Amount of unknown substance in moles
Given amount
in moles
Moles of
unknown from
equation
Moles of given
from equation
(mole ratio)
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= unknown
amount in moles
Moles to mass
• Given in moles
• To
• Unknown in moles
• To
• Unknown in grams
Given amount
in moles
Moles of
unknown from
equation
Moles of given
from equation
(mole ratio)
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Grams of
unknown
1 mole of
unknown
= unknown
amount in grams
(molar mass)
Mass to moles
• Given in grams
• To
• Given in moles
• To
• Unknown in moles
Given amount
in grams
1 mole of given
Grams of given
(molar mass)
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Moles of
unknown from = unknown
amount in moles
equation
Moles of given
from equation
(mole ratio)
Mass to mass
• Given in grams
• To
• Given in moles
• To
• Unknown in moles
• To
• Unknown in grams
Given amount
in grams
1 mole of given
Grams of given
(molar mass)
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Moles of
unknown from
equation
Moles of given
from equation
Grams of
unknown
1 mole of
unknown
= unknown
amount in grams
(mole ratio) (molar mass)
Carry units through
• All the way through the problem
• g and mol
• Elements or compounds
– This saves a lot of confusion
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Moles to moles - example
• The elements lithium and oxygen react
to form lithium oxide, Li2O. How many
moles of lithium oxide will form if 2.0
mol of lithium react?
• Li + O2  Li2O
• 4Li + O2  2Li2O
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You try
• The disinfectant hydrogen peroxide
H2O2 decomposes to form water and
oxygen gas. How many moles of O2 will
result from the decomposition of 5.0
mol of hydrogen peroxide?
• H2O2  H2O + O2
• 2H2O2  2H2O + O2
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Moles to mass
• When sodium azide is activated in an
automobile airbag, nitrogen gas and
sodium are produced according to the
equation:
• 2NaN3(s)  2Na(s) + 3N2(g)
• If 0.500 mol of NaN3 react, what mass
in grams of nitrogen would result?
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You try
• Coal can be converted to methane gas
by a process called coal gasification.
The equation for the reaction is:
• 2C(s) + 2H2O(l)  CH4(g) + CO2(g)
• What mass in grams of carbon is
required to react with water to form
1.00 mol CH4?
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You try
• Using the previous reaction, what mass
in grams of water is required to produce
1.00 mol CH4?
• 2C(s) + 2H2O(l)  CH4(g) + CO2(g)
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Mass to moles
• Chlorine gas can be produced commercially
by passing an electric current through a
concentrated solution of sodium chloride.
• 2NaCl(aq) + 2H2O(l)  2NaOH(aq) +
Cl2(g) + H2(g)
• If the solution contains 250 g of NaCl, how
many moles of Cl2 can be produced?
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You try
• Using the previous reaction, how many
moles of H2 can be produced from
250 g of NaCl?
• 2NaCl(aq) + 2H2O(l)  2NaOH(aq) +
Cl2(g) + H2(g)
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Mass to mass
• Sodium peroxide reacts vigorously with
water to produce sodium hydroxide and
oxygen.
• 2Na2O2(s) + 2H2O(l)  4NaOH(aq) +
O2(g)
• What mass in grams of O2 is produced
when 50.0 g of Na2O2 react?
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You try
• Using the previous reaction, what mass
in grams of water is needed to react
with 50.0 g of Na2O2?
• 2Na2O2(s) + 2H2O(l)  4NaOH(aq) +
O2(g)
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You try
• Milk of magnesia, a suspension of
Mg(OH)2 in water, reacts with stomach
acid, HCl.
• Mg(OH)2(s) + 2HCl(aq)  2H2O(l) +
MgCl2(aq)
• What mass in grams of HCl is required
to react with 3.00 g of Mg(OH)2?
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Limiting reactant
• AKA limiting reagent
• The reactant that runs out first in a
reaction
• Limits the amounts of other reactants
that will be used
– And the amounts of products that will be
produced.
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Examples
• In a game of musical chairs, what is the
limiting reactant?
– Number of chairs
• If you have 8 hot dog buns and 10 hot
dogs, what is the limiting reactant?
– Number of buns
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Excess reactant
• Not completely used up in a reaction
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Example
• Methanol is synthesized according to
the following equation. If 500 mol of
CO and 750 mol of H2 are present,
which is the limiting reactant? How
many moles of methanol are produced?
• CO(g) + 2H2(g)  CH3OH
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You try
• Zinc citrate is synthesized according the
following reaction. If there are 6 mol of
ZnCO3 and 10 mol of C6H8O7, which is
the limiting reactant? How many moles
of Zn3(C6H5O7)2 will be produced?
• 3ZnCO3(s) + 2C6H8O7(aq) 
Zn3(C6H5O7)2(aq) + 3H2O(l) + 3CO2(g)
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Example
• Aspirin is synthesized by the reaction of
salicylic acid with acetic anhydride.
When 20.0 g of C7H6O3 and 20.0g of
C4H6O3 react, which is the limiting
reagent? What mass in grams of
aspirin is formed?
• 2C7H6O3 + C4H6O3  2C9H8O4 + H2O
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You try
• The unbalanced equation for the
reaction of calcium phosphate with
sulfuric acid is as follows:
• Ca3(PO4)2 + H2SO4  CaSO4 + H3PO4
• If 250 g of Ca3(PO4)2 react with 3 mol
of H2SO4, will 3 mol of CaSO4 be
formed? Why or why not?
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Theoretical yield
• The maximum amount of a product that
can be produced from a given amount
of reactant.
• “in theory” how much you can get
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Actual yield
• Always less than theoretical yield
• The measured amount of product
• What you actually get
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Percent yield
• Shows efficiency of a reaction
• Ratio of actual yield to theoretical yield
actual yield
percent yield 
100 (to get %)
theoretica l yield
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You try
• The calculated theoretical yield for a
reaction is 87.2 g.
• The measured actual yield is 82.3 g.
• What is the percent yield?
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Example
• Huge quantities of sulfur dioxide are
produced from zinc sulfide by means of
the following reaction. If the typical
yield is 86.78%, how many grams of
SO2 should be expected if 4897 g of
ZnS are used?
• 2ZnS + 3O2  2ZnO + 2SO2
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