The mole concept

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The mole concept
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Lets introduce you to a common unit in
chemistry the mole (mol)

• The mole is a unit, just like the gram.
• It is used to measure the amount of substance
  present
• Used to convert numbers of molecules to grams or
  liters
• Used in stoichiometry

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Who discovered it?

• Founded by Avogadro
• The conversion factor is
• 1 mole 6.02 x 1023 molecules
• We used this conversion factor in unit one, for
  this unit we are going to focus on the unit mole,
  not really the conversion factor

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How big is a mole?

• A mole is equal to every gram of sand on every
  beach and ocean floor on the planet
• If you were given 1 mole of dollars the day you
  were born and then spent a billion dollars a
  second for your entire life before dieing at age
  70, you would have only spent 0.001 of your
  money before you died

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What do we use the mole for?

• Finding the molar mass of a compound

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Molar Mass

• The molar mass of any substance is the mass of
  one mole of particles of that substance (in
  grams)
• Also know as the gram formula mass (gfm)

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Molar Mass

• Steps Example Mg3N2
• List the elements you are looking at
• Multiply the atomic mass of each element by the
  number of atoms of that element present
  (subscripts)
• Add all of the masses together

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Lets do an example

• Calculate the molar mass of the following
• H2
• H2O
• NaCl
• Mg3(PO4)2

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Mole-gram conversions

• m M x n
• m mass in grams
• M molar mass (g/mol)
• N amount in moles

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Percent composition

• Now that we know molar mass, lets use it to help
  us identify the percent composition of various
  compounds

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Percent composition

• How much of a substance is made up from the
  different parts
• The percent composition (percentage composition)
  of a compound is a relative measure of the mass
  of each different element present in the
  compound.

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Formula for percent composition of mixtures

• Mass of part
• --------------------------- x 100
• Total mass of mixture

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Example

• You have a mixture of sugar and sand that weighs
  5 g. If 3 g of the mixture is sugar, what
  percent of the mixture is sugar?

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You try one

• You have a mixture of sugar and water. The
  mixture has a total mass of 100 g. If the
  mixture is made up of 5 g of sugar and 95 grams
  of water, then what is the percent composition of
  each part of the mixture?

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Steps example composition of water

• Calculate the molar mass compound
• Divide the mass of each element in the compound
  by the molar mass
• Multiply each answer to number 2 by 100.

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Example - NaCl

• Step 1 Na
• Cl

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Science Bell Work

• Calculate the percent composition of the
  following
• CO2
• C6H12O6

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Examples

• Calculate the percent composition of the
  following
• C6H12O11
• SO2
• CO
• SO
• NO

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Bell Work

• Calculate the percent composition of
• Copper (IV) oxide
• Gold (I) bromide
• Sulfur trioxide

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Review

• Get out a sheet of paper and make four columns
  (we are making a data table)
• Column 1 name
• Column 2 formula
• Column 3 molar mass
• Column 4 percent composition

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It should look something like this
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Continued
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Lets go the other way Empirical formula

• Definition The empirical formula is the simplest
  formula for a compound
• What if you are given the percent composition and
  asked to write the formula?
• Example What molecule is composed of 27 Carbon
  and the rest is oxygen?

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Step 1

• Write down what you know
• 27 Carbon
• Rest is Oxygen (so what percent is oxygen)

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Step 2

• Divide each percent by the mass of each element
• 27 C / 12 2.25
• 73 O / 16 4.5625

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Step 3

• Look at all of your answers to step 2 and decide
  which one is the smallest then divide ALL
  answers by the smallest answer
• 27 C / 12 2.25 / 2.25 1
• 73 O / 16 4.5625 / 2.25 2.027

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Step 4

• If your answers do not include a .5 in them, then
  your answers are your subscripts
• 27 C / 12 2.25 / 2.25 1
• 73 O / 16 4.5625 / 2.25 2.027
• C1O2
• However, if one or more of your subscripts ends
  in a .5, then you have to do step 5

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Step 5

• If one of your subscripts ends in a .5, then you
  have to multiply ALL of your subscripts by 2 to
  get to a whole number
• Example
• C1O2.5 2 C2O5

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Lets do some more examples

• Please calculate the empirical formula for the
  following
• 40.00 C, 6.72 H, 53.29 O

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Practice

• Get a white board
• 49.5 C, 5.2 H, 28.8 N, 16.5 O

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One more

• A compound has the following composition.
  Determine the empirical formulaCu79.9O20.1

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Bell Work

• A compound is found to have a percentage
  composition of 22.1 aluminum, 25.4 phosphorous,
  and 52.5 oxygen. Calculate the empirical formula
  for this compound, and then name the formula for
  this compound.

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Review of unit

• What is the formula for barium hydroxide
  trihydrate?
• What is the molar mass of ammonium sulfate?
• What is the percent composition of Carbon
  tetrafluoride?

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Review from yesterday

• One student in an art class was interested in the
  many different effects that ceramic glazes
  create. One glaze that gave an iridescent effect
  contained 48.8 cadmium, 20.8 carbon, 2.62
  hydrogen, and 27.8 oxygen. What is the empirical
  formula of the compound?

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Another one

• What is the empirical formula for a compound that
  is 31.9 potassium, 28.9 chlorine, and 39.2
  oxygen?

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Answers

• 1. CH4
• 2. KCl
• 3. AlPO4
• 4. MgBr2
• 5. Na2SO4
• 6. CuSO4 5H2O

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Empirical formulas

• What if you are given grams instead of percent?
• An orange compound contains 6.64 g of potassium,
  8.84 g of chromium, and 9.52 g of oxygen. Find
  the empirical formula.
• Step 1 divide the mass of each element by the
  molar mass (get each element from grams to moles)
• 6.64 g K 1 mol K
• ------------ x ----------- 0.170 mol K
• 1 39 g K
• 8.84 g Cr 1 mol Cr
• ------------ x -------------- 0.170 mol Cr
• 1 52 g Cr
• 9.52 g O 1 mol O
• ------------ x -------------- 0.595 mol Cr
• 1 16 g O

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• Step 2 divide by the smallest mole ratio

0.170 mol K 0.170 mol Cr 0.595 mol
Cr
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Then solve like before
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Same thing, but instead of they give you grams
(use the same exact steps as before)

• A compound was analyzed and found to contain 13.5
  g Ca, 10.8 g O, and 75.7 g H.  What is the
  empirical formula of the compound?

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A few more steps to these problems

• Now that we know what an empirical formula is,
  lets add a few more steps to see what the
  molecular formula is.
• Remember empirical formula was the simplest
  (reduced) formula for a molecule
• Molecular formula the true formula for a
  compound the subscripts have not been reduced

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Example

• Calculate the molecular formula for a molecule
  composed of 27 Carbon and the rest is oxygen if
  the mass of the molecule is 88 g/mol.
• Step 1 write down what you know

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Step 2

• If the empirical formula is not provided, please
  calculate it
• 27 C
• 73 O

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Step 3

• Find the molar mass of the empirical formula
• (Remember multiply the subscript of each
  element by the mass of each element and add them
  together)

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Step 4

• Divide the mass of the molecule given in the
  problem by the mass of the empirical formula that
  you calculated

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Step 5

• Multiply the subscripts of the empirical formula
  by the answer to step 4. This will give us our
  molecular formula.

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Example

• NutraSweet is 57.14 C, 6.16 H, 9.52 N, and
  27.18 O.  Calculate the empirical formula of
  NutraSweet and find the molecular formula.  (The
  molar mass of NutraSweet is 294.30 g/mol)

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One more

• A compound has an empirical formula of ClCH2 and
  a molecular weight of 98.96 g/mol. What is its
  molecular formula?

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Empirical review

• Find the empirical formula of a compound that is
  48.38 carbon, 8.12 hydrogen, and 53.5 oxygen
  by mass .

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Bell Work

• A compounds empirical formula is N2O5. If the
  molecular mass is 216.83 g/mol, what is the
  molecular formula?
• Write your answers on the white board
• Get out your study guide and review for the test