Chapter%206:%20Moles,%20Molar%20Mass,%20Percent%20Composition%20and%20Formulas

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Chapter 6 Moles, Molar Mass, Percent Composition
and Formulas

• From moles to mass and to the moon!

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AMU (Atomic Mass Units)

• The mass of Carbon-12 is 12 AMU.
• But wait, when I look on the periodic table, the
  atomic mass is listed as 12.01078 AMU??? WHY?
  Why, cruel world?
• WwWWwwhhHHHhhyyYYYYyy???

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6.1 Atoms and Moles

• Avogadros Number
• 6.022 x 1023
• Avogadro discovered that there are
• 6.022 x 1023 atoms in 1 gram of hydrogen.

Count Lorenzo Romano Amedeo Carlo Avogadro di
Quaregna e Cerreto
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Be able to explain and use the concept of the
mole

• This number is called a mole.
• The word mole
• is just like the
• word dozen. Dozen
• means 12. You can
• have a dozen of
• anything. You can
• also have a
• mole of anything.

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So How Big is a MOLE

• Ummm NO!
• Here it is written out
• 602,200,000,000,000,000,000,000
• Thats 602 billion groups of a trillion!
• Lets just do an example with paper clips.
• If you have a mole of paper clips and made them
  into a chain, how many times could you go to the
  moon and back with your chain?

Dont be cruel now
Aaaiiiee
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• Assume a paper clip ( still folded) is about 3 cm
  long.
• To find the total distance of the paper clips we
  use the following equation

Notice the unit clips cancels!!! Isnt that
Great Anyone Anyone see the greatness??? Man
I love Conversions!
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The moon is 382,171 km from Earth, so to the moon
and back would be 764,342 km.

• So we need to convert our cm into km
• oh how fun this is a metric conversion
• This of course is a 2-step conversion because
  both units have a prefix

I love conversions!
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• Were almost done!!!
• Thats 23 trillion trips!! Mole-tastic!
• Marshmallow example A bed of marshmallows
  covering the U.S. would be 776 miles deep

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Convert moles to of atoms

• How many atoms are in 3.2 mol potassium (K)?
• Remember 1 mol 6.02 x 1023 atoms
• This can be written as a conversion factor

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How do we use the Mole in chemistry?

• The atomic mass of an element is the grams of 1
  mole of that atom
• Why do chemists use moles?
• Its fun.
• Its impossible to count atoms with your hands.
• You can easily measure the mass (in grams) of a
  chemical.

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Atomic mass grams of 1 mole of this element,
Cobalt
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Convert moles of an atom to grams

• I need 2.0 moles of copper (Cu) for an
  experiment. How many grams is that?
• Atomic mass of Cu 63.55 g/mol (round to 2
  decimals)
• mol is the abbreviation of Mole I know its
  only one letter different chemists!!!

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Converting grams to moles

• I have 302 grams of silver (Ag). How many moles
  of silver do I have?
• Step 1 Atomic mass of Ag 107.87 g/mol
• Step 2 Calculate

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6.2 Molar Mass and Percent Composition

• Atomic Mass mass of one mole of an atom
• Molar Mass mass of one mole of a substance

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Calculate Molecular Weights

• Example Calculate the Molecular Weight (MW) of
  RbI2
• Step 1 Assume you have 1 mole of this molecule
  and determine how much each element weighs from
  the periodic table.
• Step 2 Determine how many of each element you
  have
• Step 3 Add all the masses together

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Step 1 Find how much each element weighs from
the periodic table

• Rb is atomic 37. How much does each mole of Rb
  weigh?
• 85.47 grams/mol Rb
• I is atomic 53. How much does it weigh?
• 126.90 g/mol I

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Step 2 Determine how many of each element you
have

• Look at the formula RbI2
• We have 1 Rb atom
• and 2 I atoms

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Step 3 Add all the masses together

• You will need to show this work
• Because the units are the same we can add these
  two numbers together, so
• 253.80 g/mol 85.47 g/mol 339.27 g/mol
• 339.27 g/mol is the molar mass

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Converting from moles of a compound to grams

• Example I need 3.00 mol NaCl for an experiment.
  How many grams is that?
• Step 1 Find the molar mass
• Molar mass 22.09g/mol 35.45g/mol
• 57.54 g/mol
• Step 2 Use the molar mass like a conversion
  factor.

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Converting from grams of a compound to moles

• Example How many moles are in 10.0 g of Na2SO4?
• Step 1 Find the molar mass.
• Molar mass 142.1 g/mol
• Step 2 Use the molar mass like a conversion
  factor. You need grams on the bottom of the
  fraction.

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6.3 Formulas of Compounds

• Calculate percent composition
• Just like any other
• Stuff grams of elements

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Calculate percent composition

• Ex calculate of Cu and S in Cu2S
• Stuff grams Cu
• (63.55 g/mol Cu)(2 mol Cu) 127.1g Cu
• Total stuff grams Cu grams S
• 127.1 g 32.07 g 159.17 g 159.2 g

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You should be able to

• Identify an empirical formula and a molecular
  formula
• Empirical formula simplest ratio of atoms of
  each element in a compound (whole s only)
• Molecular formula actual of atoms of each
  element in a compound

Molecular H2O2 C3H6 N2O3 C2H6 C3H9 Empirical HO CH
2 N2O3 ? ?
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Using composition to determine a formula

• Law of Definite Composition Any amount of a
  pure compound will always have the same ratio of
  masses for the elements that make up that
  compound
• Ex H2O is always 88.9 O and 11.1 H by mass
• Only the simplest formula (ratio) can be found
  in other words, you can only find empirical
  formulas

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Using composition to calculate the formula

• Process is as follows
• Calculate by mass of each element
• Determine mass of each element
• Easy if you use 100 g of the chemical
• Use mass to find the of moles of each element
• Find the smallest ratio of the atoms
• the number of moles of each element by the
  element with the smallest of moles
• Round to the nearest whole

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Example

• A molecule is 75 C 25 H. Calculate the
  empirical formula.
• Using 100g total 75g C and 25 g H
• Calculate moles of each
• Ratio 6.2C 25H, simplify by each by 6.2.
  Whole number only!!
• Final ratio 1C 4H so CH4

H
C
1
4
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C H
Percent 75 25
100 g total 75g 25g
Moles
6.2 mol 25 mol
Ratio
1 4
Formula CH4 CH4
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Find the molecular formula

• Ex C3H6O2 is an empirical formula for a
  chemical. The molar mass of the compound is 148
  g/mol.
• What is the molecular formula of the compound??
• Point The ratio of CHO will always be what the
  empirical formula shows
• Steps
• 1. Calculate the empirical formula mass
• 2. Calculate molar mass/empirical formula mass
• 3. Multiply your subscripts by that .

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• Steps
• 1. Calculate the empirical formula mass
• C3H6O2 mass (3)(12.01) (6)(1.01) (2)(16.00)
  
• 148.09 g/mol
• 2. Calculate (molar mass)/(empirical formula
  mass) Round to a WHOLE number.
• 3. Multiply the subscripts of the empirical
  formula by that number.
• C3x2H6x2O2x2 C6H12O4