Empirical

1
Empirical Molecular Formulas

• Unit 4 Stoichiometry
• Chapter 10 The Mole

2
Percentage Composition

• Definition
• The mass of each element in a compound compared
  to the entire mass of the compound and multiplied
  by 100 percent
• Percentage of hydrogen in water
• 2.0 g H x 100
• 18 g H2O

3
Practice

• Suppose you spend 6 hours instant messaging
  friends each day.
• What percentage of each day do you spend
  texting?
• 6 24 0.25 hours
• 0.25 x 100 25

4
More Practice

• Find the percentage composition of a compound
  that contains 1.94 g of carbon, 0.48 g of
  hydrogen, and 2.58 grams of sulfur in a 5.00 g
  sample of that compound.
• C (1.94 g 5.00 g) x 100 38.8 carbon
• H (0.48 g 5.00 g) x 100 9.6 hydrogen
• S (2.58 g 5.00 g) x 100 51.6 sulfur

5
Empirical Formula

• Definition
• A formula that gives the simplest whole-number
  ratio of the atoms of elements

6
Determining the Empirical Formula

• Now that you know how to calculate percentage
  composition, you can use that ratio of masses to
  find the ratio of atoms which is a chemical
  formula
• Ratio of Masses ? Ratio of Atoms
• ( composition) ? chemical formula

7
Steps for Finding Empirical Formula

• Percentage ? Mass (g)
• Mass (g) ? of Moles
• Once you have moles, find the smallest whole
  number ratio between the moles.

8
Example

• Determine the empirical formula of a compound
  containing 5.75 g Na, 3.5 g N, and 12.0 g O.
• Since you have mass, convert mass ? moles
• Na 5.75 g Na x 1 mol Na 0.250 mol
• 22.99 g
• N 3.5 g N x 1 mol N 0.25 mol
• 14.00 g
• O 12.0 g O x 1 mol O 0.75 mol
• 15.99 g

9
Example Continued

• Now divide each mole value by the smallest number
  of moles
• Na 0.25/0.25 1
• N 0.25/0.25 1
• O 0.75/0.25 3
• Empirical Formula Na N O3

10
Molecular Formula

• Definition
• The formula that gives the actual number of atoms
  of each element in a molecular compound
• The molecular formula is always a whole-number
  multiple of the empirical formula.

11
Determining Molecular Formula

• Determine molecular formula by comparing the
  molar mass of an unknown compound with the molar
  mass of the empirical formula.

12
Example

• Find the molecular formula of a compound that
  contains 42.56 g of palladium and 0.80 g of
  hydrogen. The molar mass of the compound is
  216.8 g/mol.
• Convert to moles
• 42.56 g Pd x 1 mole Pd 0.40 mol Pd
• 106.42 g
• 0.80 g H x 1 mole H 0.79 mol H
• 1.01 g

13
Example Continued

• Now find mole ratio
• 0.40/0.40 1 0.79/0.40 2
• Now write empirical formula
• Pd H2

14
Example Continued

• We know that the molar mass of the compound
  216.8 g/mol
• What is the empirical molar mass?
• If empirical formula is PdH2 , then the
  empirical molar mass 106.42g/mol Pd
  2(1.01g/mol H) 108.44 g/mol

15
Continued Once Again

• Now take the Molar Mass
• Empirical Formula Mass
• 216.8 g/mol 1.995 ? 2
• 108.44 g/mol
• FINALLY!!! Now adjust the molar formula
• PdH2 becomes Pd2H4