Stoichiometry

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Stoichiometry

• Mass Calculations

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• Yesterday Mole to Mole Ratios relating
  reactants and products in a chemical equation
• If we have a balanced chemical equation, we can
  calculate the number of moles of a substance
  using the known ratio of reactants and products
  when given the number of moles of one of the
  reactants or products.
• Previous classes
• Relationship between mass (g), molar mass
  (grams/mol) and mols of a substance ? of mols
  mass / MM
• Molar Mass is the atomic mass of an element or
  formula weight of a compound (in grams/mol)
• 1 mole 6.022 X 1023 particles/atoms/molecules

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Today Mass to Mass Calculations

• If we know the reaction (from the balanced
  chemical equation) and the amount of one of the
  substances (mass, of moles, or particles) in
  the reaction, we can calculate the amount of the
  other substances in the reaction.

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Mass to Mass Calculations
The general strategy
Step 1 Write a balanced chemical equation
Step 2 If you are given the mass or number of
particles of a substance, convert it to the
number of moles.
Step 3 Calculate the number of moles of the
required substance based on the number of moles
of the given substance, using the appropriate
mole ratio
Step 4 Convert the number of moles of the
required substances to mass or number of
particles.
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3.) Calculate the number of mols of the required
substance ? use mol to mol ratio from the
balanced equation
1.) Write out the balanced chemical equation
4.) Convert the number of moles of the required
substances to a mass or number of particles ? Use
the molar mass of the required substance
2.) Convert the given mass of a substance to
moles of the substance ? Use the molar mass of
the given substance
Mass (g) of the required substance
Mass (g) of given substance
Moles
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Mass to Mass Calculations for Products and
Reactants
Example 1
Iron can be produced from iron ore, Fe2O3 by
reacting the ore with carbon monoxide (CO).
Carbon dioxide is also produced. What mass of
iron can be formed from 425g of iron ore?
Step 1 Write out the balanced chemical equation
Fe2O3 3CO ? 2Fe 3CO2
m 425g
m ?
Step 2 Convert given mass into moles
Moles of Fe2O3 425g
159.7g/mol
2.66 mol of Fe2O3
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Mass to Mass Calculations for Products and
Reactants
Step 3 Calculate the number of mols of the
required substance (mol to mole ratio)
1Fe2O3 3CO ? 2Fe 3CO2
x mols of Fe 2 mol of Fe
2.66 mols of Fe2O3 1mol of Fe2O3
5.32 mols of Fe
Step 4 Convert moles of required substance to
the mass
mass of Fe 5.32 mols of Fe X 55.85g/mol of Fe
297g of Fe
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Mass to Mass Calculations for Reactants
Example 2
What mass of hydrazine (N2H4) is required to
react completely with 1000g of dinitrogen
tetraoxide (N2O4)?
Step 1 Write out the balanced chemical equation
2N2H4 N2O4 ? 3N2 4H2O
m ?
m 1000g
Step 2 Convert given mass into moles
moles of N2O4 1000g
92.00g/mol
10.87 mol of N2O4
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Mass to Mass Calculations for Reactants
Step 3 Calculate the number of mols of the
required substance (mol to mole ratio)
2N2H4 1N2O4 ? 3N2 4H2O
x mols of N2H4 2 mols of N2H4
10.87 mols of N2O4 1 mol of N2O4
21.74 mols of N2H4
Step 4 Convert moles of required substance to
the mass
mass of N2H4 21.74 mols of N2H4 X 32g/mol
695.7 g of N2H4
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Mass to Mass Calculations for particles/molecules
Example 3
Ammonia gas react with oxygen gas to produce
water and nitrogen monoxide. How many molecules
of oxygen are required to completely react with
23.0g of ammonia?
Step 1 Write out the balanced chemical equation
4NH3 5O2 ? 6H2O 4NO
m 23.0g
m ?
Step 2 Convert given mass into moles
Mols of NH3 23.0g of NH3
17.0g/mol of NH3
1.35 mols of NH3
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Mass to Mass Calculations for particles/molecules
Step 3 Calculate the number of mols of the
required substance (mol to mole ratio)
4NH3 5O2 ? 6H2O 4NO
x mols of O2 5 mols of O2
1.4 mols of NH3 4 mols of NH3
1.75 mols of O2
Step 4 Convert moles of required substance to
molecules
of molecules of O2 1.75 mols of O2 X 6.022
X 1023 molecules of O2

1 mol
1.05 X 1024
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Practice Questions p. 244 11,12, 14 p. 246 15,
16, 18 p. 248 19, 20 p. 249 1, 4, 6, 7