Chapter 8: Atomic Masses: counting by weighing

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Chapter 8 Atomic Masses counting by weighing

• Objective To understand atomic mass and its
  experimental determination

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Atomic Mass Atomic Mass Unit

• C(s) O2 (g) CO2 (g)
• Atomic mass unit unit of mass
• 1 amu 1.66 x 10-24
• Remember C has several isotopes 12C, 13C, and
  14C.
• Average atomic mass unit12.01

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So we can calculate the mass of any of the
following

1. Calculate the mass (in amu) of a sample of Carbon
   that contains 62 atoms.
2. Calculate the mass (in amu) of a sample of iron
   that contains 15 atoms.
3. Calculate the mass (in amu) of a sample of
   aluminum that contains 75 atoms.

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Calculate the number of atoms in

• Calculate the number of copper atoms present in a
  sample that has a mass of 1779.4 amu.
• Calculate the number of argon atoms present in a
  sample that has a mass of 3755.3 amu.
• Calculate the number of sodium atoms present in a
  sample that has a mass of 1172.49 amu.

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Chapter 8.3 The Mole

• Objectives To understand the mole concept and
  Avogadros number.
• To learn to convert among moles, mass, and number
  of atoms in a given sample.

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The Mole Revisited

• A mole (mol) is the unit a chemist uses to
  describe the numbers of atoms. It is defined as
  the number to the number of carbon atoms in
  12.01 g of carbon.
• Avogadros number is this number6.022 x 1023

Average atomic mass contains 1 mol of atoms.
Just like a dozen 12 eggs.. A mole of water
6.022x 1023
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The mole Revisited
Sample A Mass1.008 g
Sample B Mass0.500 g
? Number of H atoms.
1 mol of H atoms
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Calculating Moles and Numbers of Atoms

• Calculate the number of moles of atoms and the
  number of atoms in a 25.0g sample of calcium.
• Calculate the number of moles of atoms and the
  number of atoms in a 57.7 g sample of sulfur.

0.624 mol Ca 3.76 x 1023 Ca atoms
1.80 mol S 1.08 x 1024 S atoms
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Calculating the Number of Atoms

• Calculate the number of atoms in a 23.6 mg sample
  of Zinc.
• Calculate the number of atoms in a 128.3 mg
  sample of silver.

2.17 x 1020 Zinc atoms
7.16 x 1020 Ag atoms
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8.4 Molar Mass

• To understand the definition of molar mass
• To learn to convert between moles and mass of a
  given sample of a chemical compound.

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Mass of 1 mol of C 1 x 12.01 g12.01 g Mass of 4
mol of H 4 x 1.008 g 4.032 g Mass of 1 mol of
CH4 16.04 g.
Molar Mass of any substance Is the mass ( in
grams) of 1 mol of the substance.
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Calculate the molar mass

• Water H2O
• Ammonia NH3
• Propane C3H8
• Glucose C6H12O6

18.02 g/mol 17.03 g/mol 44.09 g/mol 180.2 g/mol
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Calculating Mass from Moles

• Calculate the mass of 1.48 mol of potassium oxide
  (K2O)
• Calculate the mass of 4.85 mol of acetic acid,
  HC2H3O2.

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Calculating Moles from Mass

• Formaldehyde has the formula H2CO. How many
  moles of formaldehyde does a 7.55 g sample
  represent?
• How many moles of tetraphosphorous decoxide does
  a 250.0 g sample represent?

0.251 mol
0.8805 mol
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Calculating Number of Molecules

• How many water molecules are in a 10.0g sample of
  water?
• Sucrose or table sugar has the formula C12H22O11.
  How many molecules of sugar are in a 5.00 lb bag
  of sugar?

3.34 x 1023
3.99 x 1024
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8.5 Percent Composition of Compounds

• Objective To learn to find the mass percent of
  an element in a given compound.

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Percent Composition

• Mass fraction
• for a given element mass of the element present
  in 1 mol of compound

Mass of 1 mol of compound
Mass percent mass fraction x 100
Mass of C 2 mol x 12.01 g/mol 24.02 Mass of H
6 mol x 1.008 g/mol 6.048 Mass of O 1 mol x
16.00 g/mol 16.00 g
C2H5OH
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Compounds containing H2O

• The crystals of a hydrate are made of solid
  substances combined chemically w/ water.
• Sodium carbonate decahydrate
• Na(CO3). 10 H2O
• First find the weight of NaCO3
• Find the weight of 10 molecules of water
• (Mass of water/ mass of hydrate) x 100

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Percent CompositionMass Percent

1. Determine the mass percent of each element in
   sulfuric acid (H2SO4)
2. Rubbing alcohol is an aqueous solution of
   isopropyl alcohol. Isopropyl alcohol has the
   formula C3H7OH. Determine the mass percent of
   each element in isopropyl alcohol.

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8.6 Formulas for Compounds

• Objective To understand the meaning of the
  empirical formula of compounds.

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Empirical Formulas

• Empirical formula smallest whole number ratio of
  the atoms. (simplest formula)
• Molecular formula actual formula
• (CH2O)6 C6H12O6
• Give the empirical formula for the following
• H2O2 hydrogen peroxide
• C4H10 butane

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Calculation of Empirical Formulas

• To learn to calculate empirical formulas

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Calculation of Empirical Formulas
Total mass of nickel oxide
Mass of nickel originally present
Mass of oxygen that reacted with the nickel

0.3354 g - 0.2636
0.0718 g Calculate the amount
of moles 0.2636 g Ni x 1mol Ni atoms 0.00491
58.69 g Ni 0.0718 g O x 1
mol O atoms 0.00449 mol O atoms
16.00 g O
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Calculation of Empirical Formulas
Steps for Determining the Empirical Formula of a
Compound STEP 1 Obtain the mass of each element
present (in grams). STEP 2 Determine the number
of moles of each type of atoms present. STEP 3
Divide the of moles of each element by the
smallest of moles to convert the
smalles to 1. If all of the s so obtained are
integers, they are the subscripts in the
empirical formula. If one or more are not
integers, go to STEP 4 Multiply the s you
derived in step 3 by the smallest integer that
will convert all of them to whole
numbers. This set of whole numbers represents
the subscripts in the empirical formula.
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Calculation of Empirical Formulas
A 1.500 g sample of a compound containing only
carbon and hydrogen is found to contain 1.198 g
of carbon. Determine the empirical formula for
this compound. 2) A 3.450 g sample of Nitrogen
reacts w/ 1.970 g of oxygen. Determine the
empirical formula for this compound.
CH3 N2O
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Calculation of Empirical Formulas for Binary
Compounds

• When a 2.000 g sample of iron metal is heated in
  air,
• it reacts with oxygen to achieve a final mass of
  2.573 g.
• Determine the empirical formula for this Iron
  oxide.
• 2) A 4.550 g sample of cobalt reacts with 5.475 g
  chlorine to
• form a binary compound.
• Determine the empirical formula for this compound.

FeO iron (II) oxide CoCl2
cobalt(II)chloride
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8.8 Calculation of Molecular Formula

• Objective To learn to calculate the molecular
  formula of a compound, given its empirical
  formula and molar mass.

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Molecular Formula

• Molecular formula (empirical formula)n
• Molar mass n x empirical formula
• So to solve for n molar mass
• Empirical formula
  mass

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8.8 Molecular Formula

• Example A white powder is analyzed and found to
  have an empirical formula of P2O5. The compound
  has a molar mass of 283.88 g. What is the
  compound molecular formula?
• Empirical formula weight Molar Mass
• 2 P 2 x 30.97 g 61.94 g 283.88
  g 2
• 5 O 5 x 16.00 g 80.00 g 141.94 g
• 141.94 g
• MOLECULAR FORMULA is P4O10

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EXAMPLES

• A compound containing carbon, hydrogen, and
  oxygen is found to be 40.00 carbon and 6.700
  hydrogen by mass. The molar mass of this
  compound is between 115 g/mol and 125 g/mol.
  Determine the molecular formula for this compound.

C4H8O4
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Example 2

• Caffeine is a compound containing carbon,
  hydrogen, nitrogen, oxygen. The mass percent
  composition of caffeine is 49.47 carbon, 5.191
  hydrogen, 28.86 nitrogen and 16.48 oxygen. The
  molar mass of caffeine is approximately 194
  g/mol. Determine the molecular formula for
  caffeine.

C8H10N4O2