The Mole: Avogadro

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The Mole Avogadros number
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How much is

• A dozen?
• A century?
• A mole?

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Can you count a mole of pennies?

• If you could count 5 per second, it would take
  you
• 6.02 x 10 23 5 pennies/second 60 sec/min 60
  min/hr 24hrs/day 365 days/yr

4
Can you spend a mole of dollars?

• If you could spend 1,000,000 every second it
  would take you
• 6.02 x 10 23 1,000,000/sec 60 sec/min 60
  min/hr 24hrs/day 365 days/yr

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When measuring amounts, you can count or you can
mass them.
If I want 2 dozen baseballs, I can count
baseballs Or I can mass of baseballs.
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Since we cant count a mole of atoms, we MUST
mass chemicals to measure moles
6.02 x 10 23 atoms of sulfur 32.07 grams of sulfur
6.02 x 10 23 atoms of carbon 12.01 grams of carbon
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How do we measure moles?

• mole number of particles equal to the number of
  atoms in 12 g of C-12
• 1 atom of C-12 weighs exactly
• 1 mole of C-12 weighs exactly
• The number of particles in 1 mole is called
  ____________ Number 6.0221421 x 1023
• 1 mole of C atoms weighs and has
• atoms
  
• the average mass of a C atom is 12.01 amu

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How do we measure moles?

• The atomic mass on your periodic table is the
  mass of a mole of atoms of that element.
• What is the mass of a mole of copper atoms?
• So, to count 6.02 x 1023 copper atoms, we mass
  out on the scale.

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Mole and Mass Relationships
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Find the mass of

• A mole of silicon atoms
• 6.02 x 1023 atoms of nitrogen
• 6.02 x 1023 atoms of sodium
• 2 moles of sodium atoms

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How many atoms are in

• A mole of silicon
• 14.01 g of nitrogen
• 2 moles of sodium
• 45.98 g of sodium

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How many things are in

• A mole of footballs
• A mole of water
• 2 moles of pencils
• ½ mole of lead

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Molar mass is the mass of one mole of a substance

• To find the molar mass of an element, look on the
  periodic table.
• To find the molar mass of a compound, add
  all the masses of its elements

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Chemical Formulas as Conversion Factors

• 1 spider ? legs
• 1 chair ? legs
• 1 H2O molecule ? H atoms O atom

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Molar Mass of Compounds

• the relative weights of molecules can be
  calculated from atomic weights
• Formula Mass 1 molecule of H2O
• since 1 mole of H2O contains 2 moles of H and 1
  mole of O
• Molar Mass 1 mole H2O

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Find the molar mass of

• Ammonium phosphate
• NH4 PO43-
• (NH4)3PO4
• Carbon dioxide
• CO2

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Find the molar mass of

• Hydrogen gas
• H2
• Elemental hydrogen
• H

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Find the molar mass of

• Iron
• Fe
• Iron (III) hydroxide
• Fe3 OH-
• Fe(OH)3

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Converting to and from moles.

• To convert between moles and particles, simply
  multiply or divide by Avogadros number.
• 2 mol x (6.02 x 1023 particles/mol) 1.20 x 1024
  particles
• 3.1 x 1024 particles x (1 mol/ 6.02 x 1023
  particles) 5.0 mol

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Remember unitfactors?
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Converting to and from moles.

• Converting between moles and mass requires the
  molar mass of the substance from the periodic
  table.
• Element Ag g/mol
• Ionic compound CaCl2 g/mol
• Covalent compound NO2 g/mol
• Always keep at least two decimal place on all
  values taken from the periodic table.

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Converting to and from moles.

• To convert from moles to grams, multiply by molar
  mass
• 0.500 mol H2O x (18.0g/mol)
• 9.00g H2O
• To convert from grams to moles, divide by molar
  mass
• 54g H2O x (1mol/18.0g)
• 3.0 mol H2O

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Converting to and from moles.

• For gases, use the fact that at STP, 1 mol of any
  gas has a volume of Liters.
• What is STP?
• Standard Temperature K or C
• Standard Pressure atmosphere
• mm Hg (barometric)
  kPa.

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Converting to and from moles.

• To go from moles to volume, multiply by 22.4L.
• 3.00 mol x (22.4L/mol) of gas
• To go from volume to moles, divide by 22.4L
• 44.8L x (1mol/22.4L) moles of gas

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Converting to and from moles.

• A convenient tool for making these conversions is
  called a mole map.
• With the mole at the center, we can put all of
  the aforementioned calculations together into one
  simple picture.

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The Mole Map
Gas Volume _at_ STP
Particles
Mole
Mass
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Percent Composition

• Percentage of each element in a compound
• By mass
• Can be determined from
• the of the compound
• the experimental mass analysis of the compound
• the total mass of
• The percentages may not always total to 100 due
  to

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What percentage of water is Oxygen?

• Formula of the compound
• Mass of the compound
• Mass of each element

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Mass Percent as a Conversion Factor

• the mass percent tells you the mass of a
  constituent element in 100 g of the compound
• the fact that NaCl is 39 Na by mass means that
  100 g of NaCl contains 39 g Na
• this can be used as a conversion factor
• 100. g NaCl ? 39.0 g Na

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Empirical Formulas

• The simplest, whole-number ratio of atoms in a
  molecule is called the
• can be determined from percent composition or
  combining masses
• The Molecular Formula is a multiple of the
  Empirical Formula

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Empirical Formulas
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Finding an Empirical Formula

• convert the
• skip if already grams
• convert
• use molar mass of each element
• divide all by
• round or multiply all mole ratios by number to
  make all whole numbers
• if ratio ?.5, multiply all by 2 if ratio ?.33 or
  ?.67, multiply all by 3, etc.
• skip if already whole numbers

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• Determine the empirical formula of a compound
  containing 80.0 grams of carbon and 20.0 grams
  hydrogen.

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Grams to moles
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Divide by smallest
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Write Empirical Formula
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• Example
• A laboratory analysis of aspirin determined the
  following mass percent composition. Find the
  empirical formula.
• C 60.00
• H 4.48
• O 35.53

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All these molecules have the same Empirical
Formula. How are the molecules different?
Name Molecular Formula Empirical Formula
glyceraldehyde C3H6O3 CH2O
erythrose C4H8O4 CH2O
arabinose C5H10O5 CH2O
glucose C6H12O6 CH2O
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Molecular Formulas

• The molecular formula is a multiple of
• To determine the molecular formula you need to
  know and the
  of the compound

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What is the molecular formula for ethane if it
has a molar mass of 30.0 g/mol?

• CH3 g/mol
• Molecular formula

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Determine the Molecular Formula of Cadinene if it
has a molar mass of 204 g and an empirical
formula of C5H8
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Solutions
0

• Ask a chemist, they always have

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Solutes and Solvents
0

• Solution
• Solute
• Solvent
• (usually found in the largest amounts)
• If the solvent is water, then it is called an

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Solubility
0

• Why does sugar disappear in your iced tea?
• How do fish breathe underwater?
• Why does soda go flat faster when left out than
  when it is refrigerated?
• It is all based on solubility!

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Solubility
0

• Example iced tea
• Solute
• Solvent

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States and Solutions
0

• Solutions can be any state of matter
• Solid-solid
• Solid-liquid
• Liquid-liquid
• Liquid-gas
• Gas-gas

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How Things Dissolve
0

• Need to find/ create __________ in water for
  the dissolving substance to move
• Get _______________ between water molecules and
  molecules of the solute

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Why some coffee is so strong it can Put hair on
your chest.
0

• Strong coffee has more coffee dissolved in a
  given amount (say 1 pot) than weak coffee.
• Strong coffee concentrated
• Weak coffee dilute
• Concentration

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Molarity (M)
0

• Most common way to express concentration
• Molarity is the number of ____________ of solute
  dissolved in each __________of solution
• Formula
• M moles of solute
• liters of solution
• Dependent on _______________
• The higher the molarity the stronger the
  concentration

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Practice Problems

• 1. What is the molarity when 6.0 moles of glucose
  is dissolved in water to make 3.0 L of solution.

2. How many moles of sodium chloride are there in
500 mL of 4.0 M solution?
3. What is the volume of 3.0 M solution that
contains 15 moles of glucose?
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How does something so strong become so weak?
0

• The answer is dilution.
• The more dilute something is, the lower the
  concentration (its weaker).
• To accomplish this, add more solvent
• How do we know how much to add?
• M 1V1 M 2V2
• Typically start with a highly concentrated
  solution and dilute down to what you need

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Figure 15.8 Process of making 500 mL of a 1.00
M acetic acid solution.
0
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End of Chapter