Molecular formulas

1
Molecular formulas

2
Review of Moles

• Avogadros number
• 6.022 x 1023 Particles per mole of a substance
• The particles can be atoms, molecules or ions
• Molar Mass
• The mass of one mole of a substance

3
Review Average Atomic Mass

• weighted average of the atomic mass of an
  elements isotopes
• Found by
• taking into account the abundance of an atom in
  the environment

4
Formula Mass link

• Formulas for covalent compounds show the elements
  and the number of atoms of each element in a
  molecule.
• Formulas for ionic compounds show the simplest
  ratio of cations and anions in any pure sample.

5
Empirical vs actual vs molecular formulas

• empirical formula
• shows the simplest ratio for the relative numbers
  and kinds of atoms in a compound.
• ammonium nitrate is NH2O
• actual formula
• shows the actual ratio of elements or ions in a
  single unit of a compound.
• NH4NO2
• molecular formula
• is a whole-number multiple of the empirical
  formula

6

7
Determining Empirical Formulas

• You can use the percentage composition for a
  compound to determine its empirical formula.
• Convert the percentage of each element to g.
• Convert from g to mol using the molar mass of
  each element as a conversion factor.
• Compare these amounts in mol to find the simplest
  whole-number ratio among the elements.

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Determining an Empirical Formula from Percentage
Composition
Sample Problem G Chemical analysis of a liquid
shows that it is 60.0 C, 13.4 H, and 26.6 O by
mass. Calculate the empirical formula of this
substance. link
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Determining an Empirical Formula from Percentage
Composition
Sample Problem G Solution Assume that you have a
100.0 g sample, and convert the percentages to
grams. for C 60.0 ? 100.0 g 60.0 g C for
H 13.4 ? 100.0 g 13.4 g H for O 26.6 ?
100.0 g 26.6 g O
10
Determining an Empirical Formula from Percentage
Composition
Sample Problem G Solution, continued Convert the
mass of each element into the amount in moles,
using the molar mass.
11
Determining an Empirical Formula from Percentage
Composition
Sample Problem G Solution, continued The formula
can be written as C5H13.3O1.66, but you divide by
the smallest subscript to get whole numbers.
The empirical formula is C3H8O.