The Mole

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The Mole

• Pay Attention this is really important!

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What is The Mole?

• Its not this

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Or this
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• And were not talking about food either

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• Were talking about this

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The Mole Defined

• The mole is the standard method in chemistry for
  communicating how much of a substance is present.
• Here is how the International Union of Pure and
  Applied Chemistry (IUPAC) defines "mole"

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• The mole is the amount of substance of a system
  which contains as many elementary entities as
  there are atoms in 0.012 kilogram of carbon-12.
  When the mole is used, the elementary entities
  must be specified and may be atoms, molecules,
  ions, electrons, other particles, or specified
  groups of such particles. This is the fundamental
  definition of what one mole is. One mole contains
  as many entities as there are in 12 grams of
  carbon-12 (or 0.012 kilogram).

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• In one mole, there are 6.022 x 1023 atoms. Here's
  another way there are 6.022 x 1023 atoms of
  carbon in 12 grams of carbon-12
• Holy Smokes! That is a lot of zeroesand that
  number actually has a name.

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Avogadros Number

• Never call it Avocados Number. Remember, were
  not talking about guacaMOLE here, just the mole.

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• 6.022 x 1023 is such an important number that it
  has its own name and symbol, N.
• Named Avogadros Number in honor of Italian
  chemist who made critical contributions to
  measuring atomic weights.

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The Mole Applied

• In one mole of ANYTHING there are 6.022 x 1023
  entities.
• So
• One mole of donuts contains 6.022 x 1023
  donutsOne mole of carbon atoms contains 6.022 x
  1023 of carbon atomsOne mole of water
  molecules contains 6.022 x 1023 of H2O
  molecules.

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Molecular Weight (Mass)

• Molecular Weight weight in atomic mass units of
  all the atoms in a given formula.
• Symbol was and is most commonly amu but not the
  most correct symbol is u.
• Why do we need to know molecular weight? It is
  needed in order to know how many grams are in one
  mole of a substance.

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Four Steps to Calculating a substances molecular
weight

• Determine how many atoms of each different
  element are in the formula.
• Look up the atomic weight of each element in a
  periodic table
• Multiply step one times step two for each element
• Add the results of step three together and round
  off as necessary.

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Practice

• KCl1. K 1 Cl 12. K 39.10 Cl 35.453.
  1(39.10) 1(35.45) 4. 74.55 amu represents
  the molecular weight of KCl

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More Practice

• H2O2
• H 2 O 2
• H 1.00 O 15.99
• 2(1) 2(15.99)
• 232 34 amu

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Molar Mass

• Counting atoms or molecules is very difficult
  since they are so small BUT you can count atoms
  or molecules by weighing large amounts of them on
  a balance.
• When we weigh one mole of a substance on a
  balance, this is called molar mass and has the
  units g/mol (grams per mole).

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Molar Mass

• A molar mass is the weight in grams of one mole
• One mole contains 6.022 X 1023 entities.Therefore
  , a molar mass is the mass in grams of 6.022 X
  1023 entities

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How to Calculate Molar Mass

• How do YOU calculate molar mass? The molar mass
  IS the molecular weight (!) in grams (not amus)
• So ?You calculate the molecular weight and stick
  the unit g/mol after the number and that is the
  molar mass for the substance in question.

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Example of Molar Mass

• Al(NO3)3
• (1xAl)(3xN)(9xO) ? (1x 26.98)(3x14.007) (9x
  16.00) 213.00 g/mol
• 213 grams is the mass of one mole of aluminum
  nitrate
• 213 trams of aluminum nitrate contains 6.022 x
  1023 entities of Al(NO3)3

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Practice

• AlCl3

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Converting Moles to Grams

• The mole is the standard measurement amount in
  chemistry and when substances react with one
  another they do so in ratios of moles. Problem
  isbalances give readings in grams, not moles!
• So, when comparing one substance to another using
  moles.we have to convert moles to grams since
  this is the unit we measure from balances.

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The Steps

• 3 steps to converting moles of a substance to
  grams
• Determine how many moles are given in the problem
• Calculate the molar mass of the substance
• Multiply step one by step two.

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The Equation

• The equation looks like this
• Grams of the substance molar mass of the
  substance in gramsmoles of the substance one
  mole
• You replace the grams of the substance with X
  because this is what you are solving for.

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Example Problem

• Problem Calculate how many grams are in 0.700
  moles of H2O2
• Determine how many moles you have ---- 0.700
  moles
• Determine the molar mass of the substance ----
  H2O2 has a molar mass of 34.0146 g/mol
• Multiply the moles given by the substances molar
  mass0.700 mole x 34.0146 g/mol 23.8 grams

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Significant Figures

• The answer of 23.8 trams has been rounded to
  three significant figures because the 0.700 value
  had the least number of significant figures in
  the problem.
• Using the equation this problem would look
  like x 34.0146 g 0.700 mol 1.00
  mol(then cross multiply and divide to solve for
  x)

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More practice

• Problem 2 Convert 2.50 moles of KClO3 to grams.

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Converting Grams to Moles

• The mole is the standard measurement of amount.
  However, remember that balances do NOT give
  readings in moles, they give readings in grams.
  Oh NO!!
• Steps to convert grams of a substance to moles
• Determine how many grams are given in the problem
• Calculate the molar mass of the substance.
• Divide step one by step two.

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This might look familiar

• Here is the equation
• Grams of the substance molar mass of the
  substance in gramsmoles of the substance one
  mole
• Where the moles of the substance will be the
  unknown (x) and what you solve for.

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Lets Practice

• Convert 25.0 grams of KMnO4 to moles
• Determine how many grams are present --- 25.0
  grams
• Determine the molar mass of the substance ---
  KMnO4 is 158.034 g/mol
• Divide the grams given by the substances molar
  mass25.0 g 0.158 mol158.034 g/mol

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• If this problem were set up like the proportion,
  it would look like this
• 25.0 g 158.034 g x 1.00 mol
•  
• Then, you cross-multiply and divide to solve for
  X.

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And one more practice problem

• Calculate how many moles are in 17.0 grams of
  H2O2

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Calculating The Mass of One Molecule

• When calculating the mass of one molecule, do the
  following
• Calculate the molar mass of the substance
• Divide it by Avogadro's Number
• Make sure you have a periodic table and a
  calculator handy.
• the technique to calculate the mass of one atom
  of an element is exactly the same as for
  calculating the mass of one molecule of a
  compound.

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Practice problem 1

• What is the mass of one molecule of H2O?
• Step 1 - the molar mass of water is 18.015
  g/mol. This was calculated by multiplying the
  atomic weight of hydrogen (1.008) by two and
  adding the result to the weight for one oxygen
  (15.999).
• Please remember that you need the molar mass
  first when trying to find the mass of one
  molecule.

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• Step 2 - divide the substance's molar mass by
  Avogadro's Number.
• 18.015 grams/mol 6.022 x 1023 mol1  2.992 x
  1023 gramsNote that the final answer has been
  rounded to four significant figures (from 2.9915
  - note use of rounding with five rule) and that
  the unit of mole cancels.

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Practice 2

• calculate the mass (in grams) of one molecule of
  CH3COOH
• The molar mass of CH3COOH is 60.06 g/mol. The
  solution is
• 60.06 g/mol 6.022 x 1023 mol1  9.973 x 1023 g

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Acknowledgements

• Thanks to the ChemTeam for much of the valuable
  information on this power point.