Regents Chemistry

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Regents Chemistry
Topic III
Stoichiometry
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Whats a Mole???
One mole of ANY specified entity contains 6.022 x
1023 of that entity. For example

• One mole of donuts contains 6.022 x 1023 donuts
• One mole of H2O contains 6.022 x 1023 molecules
• One mole of nails contains 6.022 x 1023 nails
• One mole of Fe contains 6.022 x 1023 atoms
• One mole of dogs contains 6.022 x 1023 dogs
• One mole of electrons contains 6.022 x 1023
  electrons

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(Covalently bonded)
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These are Formula units!
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How can we measure a mole?

• A mole 6.022 x 1023 particles
• Could be a mole of an ionic compound
• contains 6.022 x 1023 formula units (made of
  ions)
• Could be a mole of a molecular compound
• contains 6.022 x 1023 molecules (made of
  elements)
• A mole of an element the average atomic mass
  listed on the periodic table
• A mole of a compound total mass of the moles in
  the compound

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Finding the Mass of a Mole of a Compound
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Example 1
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Example 2
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Summary

• Formula mass - mass of the smallest unit of an
  ionic compound sum of atomic masses in amus
• Gram Formula Mass - same as formula mass except
  in grams
• Molecular Mass same as FM except for used for
  molecular substances
• Mole 6.022 x 1023 particles of anything! Just
  know it is also the GFM of any substance

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Regents Chemistry

• Mass, Mole and Particle conversions

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The Conversion Flowchart
Number of Particles
Mass of compound
multiply by molar mass
multiply by 6.02 x 1023
divide by molar mass
Mole
divide by 6.02 x 1023
divide by 22.4 liters
multiply by 22.4 liters
Liters of Gas
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Practice mole to mass

• How many grams are in 2 moles of CuCl2?
• How many grams are in 3.5 moles of H2O

2 moles CuCl2 x 134g per mole 268 g
3.5 moles H2O x 18g per mole 63 g
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Mass to Mole

• How many moles of NO are in 67.0g of NO?
• How many moles of KCl are in 125.0g of KCl?

67.0 g NO / 30.0 g per mole 2.2 moles NO
125.0 g KCl / 74.5 g per mole 1.68 moles KCl
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Regents Chemistry

• Finding Percent Composition

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What is Percent Composition?

• Formulas represent the composition of a substance
• If we use the subscripts and atomic masses of the
  elements, we can find the percent by mass of each
  element in a substance
• Essentially, Percent Composition finds how much
  of each element is in a substance

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How do we do this????

• 1. We can add up the total molar mass of all
  elements involved in the substance
• 2. We next simply divide the total of the element
  in question by the total molar mass and multiply
  by 100

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Example 1

• Find the percent composition (in grams) of oxygen
  in potassium chlorate

2. Divide and multiply by 100
1. Find the molar mass KClO3 K - 1
mole x 39.1 grams 39.1 g Cl - 1 mole x
35.5 grams 35.5 g O - 3 moles x 16.0 grams
48.0 g
O 48.0 g
x 100
122.6 g
Molar mass 122.6 g
O 39.2
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Moles and molecules/formula units

• Moles - measured in grams
• molecules / formula units - measured in amu
  (atomic mass units)
• So grams make up moles of a substance
• and amu (atomic mass units) make up individual
  molecules/formula units
• Use the same numbers from the PT!

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Its the same process to find the of amu in a 1
molecule or formula unit!

• Find the percent composition (in amu) of oxygen
  in potassium chlorate

2. Divide and multiply by 100
1. Find the mass in amu KClO3 K - 1
atom x 39.1 amu 39.1 amu Cl - 1 atom x
35.5 amu 35.5 amu O - 3 atoms x 16.0 amu
48.0 amu
O 48.0 amu
x 100
122.6 amu
Formula mass 122.6 amu
O 39.2
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Example 2

• Find the percent composition (in amu) of water in
  sodium carbonate crystals

Na2CO3 10H2O
3. Divide H2O 180.0 amu x 100
--------------- 286.0
amu H2O 62.9
1. Formula Mass 286.0 amu
2. Total formula mass of H2O 180.0 amu

worksheet
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Regents Chemistry

• Mole Relations in Balanced Equations

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RECAP

• The Law of Conservation of Mass tells us
• Moles (and atoms) in equations must be balanced
• Since moles (and atoms) have mass, mass is also
  balanced (we did this by finding missing mass)
• So we have relationship between reactants and
  products
• Thus if we change the moles of reactants (or
  number of atoms) we change the moles / of atoms
  of products..HOW?

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Mole Relations

• Balanced Chemical Equation

2C2H6 7O2 ? 4CO2 6H2O
Moles C2H6
Moles O2
Moles H2O
Moles CO2
2 7 4 6
4 14 8 12
1 3.5 2 3
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Sample Problem

• How many moles of water will be produced from the
  complete combustion of 3.0 mol of ethane?

2C2H6 7O2 ? 4CO2 6H2O
moles ethanemoles water 26
3.0 mole C2H6 x
2.0 mole C2H6
6 mole H2O
x 9.0 mole H2O
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Finding the Mass Produced

• Sample Problem Cont..
• How many grams of water are produced

9.0 mole H2O produced
(
)
g H2O 9.0 mole H2O 18.0g H2O 162
g H2O
1 mole H2O
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Sample Problem 2

• How many moles of iron (III) oxide are produced
  from the combination of 6 moles of Fe ? How many
  grams?

4Fe 3O2 ? 2Fe2O3
6 mole Fe x mole Fe2O3 4 mole Fe
2 mole Fe2O3
x 3 mole Fe2O3 equals 480g Fe2O3