Unit/ Chapter 6: The Mole!

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Unit/ Chapter 6 The Mole!
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A. Counting by Weighing

• Objects do not need to have identical masses to
  be counted by weighing.
• Examples tickets _at_ Dave Busters, MMs in a
  bag
• What would you need to know to find out how many?
• You must know the average mass of the objects.
• To know the number of atoms in a sample what
  information is needed?
• Mass of the sample
• Average mass of the element
• Where can this be found?

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A. Counting by Weighing

• Averaging the Mass of Similar Objects

• Example What is the mass of 1000 jelly beans?
• Not all jelly beans have the same mass.
• Suppose we weigh 10 jelly beans and find

1. Now we can find the average mass of a bean.

1. Finally we can multiply to find the mass of 1000
   beans!

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A. Counting by Weighing

• Averaging the Mass of Different Objects

• Two samples contain the same number of components
  (cookies, candies, nuts or bolts) if
• The ratio of the sample masses is the same as the
  ratio of the masses of the individual components
• Example Nuts bolts
• Average mass
• Nuts 5 g
• Bolts 15 g
• So if you have 50g of nuts, what mass of bolts
  are needed?

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Counting by Weighing

• You are a bit freaked by elevators and you are on
  an elevator that holds 1200 lbs. You look at the
  other passengers and estimate their weights as
  200 lbs, 175 lbs, 125 lbs, 100 lbs, 150 lbs. You
  know you weigh 135 lbs. When the elevator doors
  open on the next floor, how many people can get
  on the elevator without going over the weight
  limit?

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B. Atomic Masses Counting Atoms by Weighing

• The number of protons determines the element
• The numbers of electrons and neutrons can change
• Different numbers of electrons than protons
  ions
• Different numbers of neutrons isotopes
• Most of an atoms mass comes from the protons and
  neutrons
• The average atomic mass for an element is the
  weighted average of the masses of all the
  isotopes of an element.
• Weighted based on the abundance of an isotope

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B. Atomic Masses Counting Atoms by Weighing

• How can you measure one atoms mass?
• Scientists created a unit to avoid using very
  small numbers.
• 1 atomic mass unit (amu) 1.66 ?10-24 g
• Example Carbon
• Isotopes 12C, 13C, 14C
• 1 C atom 12.01 amu

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B. Atomic Masses Counting Atoms by Weighing

• How many C atoms are in a sample with a mass of
  3,000 amu?

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C. The Mole
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C. The Mole

• One mole of anything contains 6.022 x 1023 units
  of that substance.
• Avogadros number is 6.022 x 1023.
• One mole of an element a mass equal to that
  elements average atomic mass (expressed in g)
• Example Carbon

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C. The Mole
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C. The Mole

• If you have 0.6 moles of carbon how many atoms do
  you have?
• If you have 47.9 g of C, how many moles do you
  have?

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A. Molar Mass

• A compound is a collection of atoms bound
  together.

• The molar mass of a compound is equal to the mass
  of each item in the compound.

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A. Molar Mass

• For compounds containing ions the molar mass is
  obtained by summing the masses of the component
  ions.

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A. Molar Mass
Calculations Using Molar Mass

• How many moles are in 276.5 g of Fe2O3?
• How many formula units or compound particles are
  present?

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Volume Moles

• Volume is the amount of space occupied by an
  object
• Measured in m3 or L
• Which will occupy more space, 1 mole of He or 1
  mole of H2 at STP?
• STP standard temperature pressure
• 1 mole of a gas occupies 22.4 L of space.
• Conversion factor 1 mol gas 22.4 L

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Volume Moles

• How many moles are present in 8,671.5 L of Ne?
• How many grams of Ne are present?
• How many particles of Ne?

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B. Percent Composition of Compounds

• Percent composition is the mass percent of each
  element in a compound

• Mass percent

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B. PERCENT COMPOSITION OF COMPOUNDS

• Given 1 mol C2H5OH. What is the percent
  composition
• Of C?
• Of H?
• Of O?

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A. Empirical Formulas

• Empirical formula of a compound is
• The simplest whole number ratio of the atoms
  present in the compound
• Ionic compound formula units are empirical
• Example NaCl , empirical?
• C6H12O6 empirical?
• The empirical formula can be found from the
  percent composition of the compound.

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B. Calculation of Empirical Formulas
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B. Calculation of Empirical Formulas

• What is the empirical formula for a compound
  containing the following?
• 0.671 mol C
• 1.342 mol H
• 0.671 mol O
• What is the empirical formula for a compound
  containing the following?
• 0.110 mol O
• 0.165 mol Al

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C. Calculation of Molecular Formulas

• Molecular formula
• The exact formula of the molecules present in a
  substance.
• The molecular formula is always an integer
  multiple of the empirical formula. Molecular
  formula (empirical formula)n
• where n is a whole number

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Formula SummaryFor the sugar glucose