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
(No Transcript)
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