III. Formula Calculations

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III. Formula Calculations

• The Mole

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A. Percentage Composition

• the percentage by mass of each element in a
  compound

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A. Percentage Composition

• Find the composition of Cu2S.

? 100
Cu
79.852 Cu
? 100
S
20.15 S
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A. Percentage Composition

• Find the percentage composition of a sample that
  is 28 g Fe and 8.0 g O.

? 100
Fe
? 100
O
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A. Percentage Composition

• How many grams of copper are in a 38.0-gram
  sample of Cu2S?

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A. Percentage Composition

• Find the mass percentage of water in calcium
  chloride dihydrate, CaCl22H2O?

? 100
H2O
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Calculations

• molar mass
  Avogadros number Grams
  Moles particles
• Everything must go through Moles!!!

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Chemical Formulas of Compounds

• Formulas give the relative numbers of atoms or
  moles of each element in a formula unit - always
  a whole number ratio (the law of definite
  proportions).
• NO2 2 atoms of O for every 1 atom of N
• 1 mole of NO2 2 moles of O atoms to every 1
  mole of N atoms
• If we know or can determine the relative number
  of moles of each element in a compound, we can
  determine a formula for the compound.

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Types of Formulas

• Empirical Formula
• The formula of a compound that expresses the
  smallest whole number ratio of the atoms present.
• Ionic formula are always empirical formula
• Molecular Formula
• The formula that states the actual number of
  each kind of atom found in one molecule of the
  compound.

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B. Empirical Formula

• Smallest whole number ratio of atoms in a
  compound

C2H6
reduce subscripts
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B. Empirical Formula

• 1. Find mass (or ) of each element.
• 2. Find moles of each element.
• 3. Divide moles by the smallest to find
  subscripts.
• 4. When necessary, multiply subscripts by 2, 3,
  or 4 to get whole s.

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B. Empirical Formula

• Find the empirical formula for a sample of 25.9
  N and 74.1 O.

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B. Empirical Formula
Need to make the subscripts whole numbers
? multiply by
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C. Molecular Formula

• True Formula - the actual number of atoms in a
  compound

CH3
empirical formula
?
molecular formula
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C. Molecular Formula

• 1. Find the empirical formula.
• 2. Find the empirical formula mass.
• 3. Divide the molecular mass by the empirical
  mass.
• 4. Multiply each subscript by the answer from
  step 3.

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C. Molecular Formula

• The empirical formula for ethylene is CH2. Find
  the molecular formula if the molecular mass is
  28.1 g/mol?

empirical mass 14.03 g/mol