Empirical and Molecular Formulas

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Empirical and Molecular Formulas
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Empirical Formula

• If we are given an unknown substance how could we
  determine its chemical formula?
• We can determine its percent composition
• Then we can determine its empirical formula the
  simplest whole-number ratio of atoms in a compound

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• However, the empirical formula does not
  necessarily give information about the number of
  atoms in a molecule (molecular formula).

Molecular Formula Empirical Formula
C2H2 CH
C4H4 CH
C6H6 CH

• The empirical formula gives the combining ratio
  in its simplest form
• The molecular formula gives the same ratio but
  with the actual number of atoms
• Note The empirical formula and the molecular
  formula can be the same in some cases. E.g. H2O,
  CH4, CO2

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Calculating Empirical Formula

• E.g. What is the empirical formula of a substance
  that is 40.0 carbon, 6.70 hydrogen, and 53.3
  oxygen by mass?

Given Assume 100 g mC 40.0 g mH 6.70 g mO
53.3 g
? The empirical formula of the substance is CH2O
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Calculating Molecular Formula

• The empirical formula tells us the simplest ratio
  of atoms in a molecule, but it does not tell us
  the actual number of atoms in a molecule
• However, if we know molar mass and the empirical
  formula of a compound we can determine its
  molecular formula
• Divide the molar mass of the compound by the
  molar mass obtained from the empirical formula
• Multiply the subscripts by this number

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• E.g. What is the molecular formula of a compound
  that has a molar mass of 34 g/mol and the
  empirical formula HO?

Given Empirical Formula HO MMcmpd 34
g/mol MMHO 1.01 16.00 17.01 g/mol
? The molecular formula of the compound is H2O2
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• E.g. What is the molecular formula of a compound
  that has a molar mass of 45.00 g/mol and the
  empirical formula CH3?

Given Empirical Formula CH3 MMcmpd 45.00
g/mol MMCH3 12.01 3(1.01) 15.04
g/mol
? The molecular formula of the compound is C3H9
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Calculating Molecular Formula

• We can calculate molecular formula if empirical
  formula and molar mass is given
• We can also calculate molecular formula if
  percent composition and molar mass is given.

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• E.g. What is the molecular formula of a compound
  with a molar mass of 58.0 g/mol and a percent
  composition of 82.5 C and 17.5 H?

Given C 82.5 H 17.5 MM 58.0 g/mol m
58.0 g (Assume 1 mole)
Mole ratio CH 410
? The molecular formula is C4H10
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Homework

• P. 186 2-6
• P. 188 1-6
• P. 193 8,9

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Hydrates

• Solid ionic compounds are formed from dehydrating
  ionic solutions
• Some of the liquid water molecules may remain
  between molecules of the solids, creating a
  hydrate
• When a hydrate is heated and water is evaporated
  it is referred to as anhydrous

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Hydrate Example

• A 50.0 g sample of a hydrate of barium hydroxide
  was heated. The anhydrous solid weighed 27.2 g.
• Determine the percent composition of water in the
  hydrate.
• Calculate the molecular formula of the hydrate.
  Name the hydrate.
• Hint Start with a BCE
• Ba(OH)2?xH2O Solve for x