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Empirical Formula, Molecular Formula

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Title: Empirical Formula, Molecular Formula


1
Empirical Formula, Molecular Formula Percentage
Composition Dr Graeme Jones
LECTURE 4
2
Empirical Formula
  • The empirical formula of a compound is the
    simplest formula which represents its composition
  • It shows the elements present and the ratio of
    the amounts of the elements present.
  • To find the empirical formula you need to work
    out the ratio of the amounts of the elements
    present.
  • Use relative atomic masses to four significant
    figures in your calculations.

3
Examples
  • Write the empirical formulas for the compounds
    containing carbon and hydrogen in the following
    ratios
  • 2 mol carbon to 3 mol of hydrogen
  • 1 mol of carbon to 1.5 mol of hydrogen
  • 0.7126 mol of carbon to 1.069 mol hydrogen
  • 18.000g of sample of a magnesium oxide was found
    by reduction to contain 10.800 g of magnesium.
    Calculate the empirical formula.
  • 9.435g of sample of a chloride was found to
    contain 3.400g of calcium. Calculate the
    empirical formula.
  • 10.237g of sample of a chloride was found to
    contain 3.528g of iron. Calculate the empirical
    formula.
  • A sample of a compound contains 91.87 g Fe, 79.11
    g S and 157.90 g O, determine the empirical
    formula.

4
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5
Molecular Formula
  • The molecular formula is a simple multiple of the
    empirical formula e.g.
  • If the empirical formula is CH2O then the
    molecular formula could be
  • CH2O, C2H4O2, C3H6O3 . CnH2nOn
  • n can be calculated by knowing the empirical
    formula mass the molar mass

6
Example
  • Benzene has the empirical formula CH and a molar
    mass of 78, what is the molecular formula?
  • Which of the following is an empirical formula
    and which a molecular formula.
  • C3H7 C3H6 C4H10

7
Calculation of Percentage Composition
  • From the formula of a compound and the relative
    atomic masses of the elements in it the
    percentage of each element in the compound can be
    determined
  • This is formally termed the Percentage
    Composition by Mass
  • This can be extended to calculate the percentage
    composition of molecules within a substance for
    example the percentage mass of water of
    crystallisation

8
Example
  • Calculate the percentage composition of
    nitroglycerine C3H5O9N3 (C12.01, H 1.008, O
    16.00, N 14.01)
  • Calculate the percentage mass of water of
    crystallisation of nickel nitrate Ni(NO3)2.6H2O

9
Other Examples
  • Calculate the mass of phosphorous in 6.78 g of
    the fertilizer ammonium phosphate (NH4)3PO4
  • Calculate the mass of ammonium phosphate
    (NH4)3PO4 that contains 6.78 g of phosphorous
  • Determine the empirical formula of a compound
    that has the percentage composition of 56.26 O
    and 43.64 P
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