Writing Empirical and Molecular Formulas

1
Writing Empirical and Molecular Formulas
2
What is an empirical formula?

• A chemical formula in which the ratio of the
  elements are in the lowest terms is called an
  empirical formula.

3
Example

• The empirical formula for a glucose molecule
  (C6H12O6) is CH2O. All the subscripts are
  divisible by six.
• C6 H12 O6
• 6 6
  6
• C H2 O

4
Exceptions

• Some formulas, such as the one for carbon
• dioxide, CO2, are already empirical formulas
• without being reduced.

5
Calculating empirical formula

• To find the empirical formula from the molecular
  formula, you must divide all the subscripts by
  the (GCF) greatest common factor.

6
Determine the empirical formulas for each of the
following molecular formulas.

• 1. C8H18..______________
• 2. H2O2______________
• 3. Hg2Cl2..______________
• 4. C3H6.______________
• 5. Na2C2O4..._____________
• 6. H2O......________________
• 7. C4H8.._______________
• 8. C4H6..________________
• 9. C7H12._______________
• 10. CH3COOH.___________

7
Finding empirical formulas using molecular mass

• Find mass (or ) of each element.
• Find moles of each element.
• Divide moles by the smallest to find
  subscripts.
• When necessary, multiply subscripts by 2, 3, or 4
  to get whole s.

8
EXAMPLE Find the empirical formula for a sample
of 25.9 N and 74.1 O.

• 25.9g 1.85 mol N 1.85mol N 1 N x 2
  N2
• 14g/mol 1.85 mol
  
  
  
• 74.1 g 4.63 mol O 4.63 mol O 2.5 O x 2
  O5
• 16 g/mol 1.85 mol
• 
  N2O5

9
PRACTICE PROBLEMS

• A substance is 36.1 by weight calcium
• and 63.9 chlorine. What is the empirical
• formula of this compound?

10

• 2. A compound is 43.4Na, 11.3C, and
• 45.3 O. What is the empirical formula for
• this compound?

11

• 3. A compound is 2.46H, 39.1S, and
• 58.5O. What is the empirical formula for
• this compound?

12
What is a molecular formula?

• A molecular formula is the true formula of a
  compound. The chemical formula for a molecular
  compound shows the actual number of atoms present
  in a molecule.

13
To find the molecular formula from the empirical
formula

• Find the empirical formula.
• Determine the empirical formula mass.
• Divide the molecular mass by the empirical
  formula mass to determine the multiple.
• Multiply the empirical formula by the multiple to
  find the molecular formula.
• MF mass n
• EF mass
• (EF)n molecular formula

14
EXAMPLE

• The empirical formula for ethylene is CH2. Find
  the
• molecular formula if the molecular mass is
• 28.1g/mol.
• C 1 x 12 12
• H 2 x 1 2
• 14g/mol empirical formula
  mass
• 28.1 g/mol 2
• 14 g/mol 
• (CH2)2 ? C2H4

15
Practice Problems

• 1. Find the molecular formula for a compound
  with a mass of 78 amu and the empirical formula
  CH.

16

• 2. Find the molecular formula for a compound with
  a mass of 82 amu and the empirical formula C3H5.

17

• 3. Find the molecular formula for a compound with
  a mass of 90 amu and the empirical formula HCO2.

18

• 4. Find the molecular formula for a compound with
  a mass of 112 amu and the empirical formula CH2.

19

• 5. Find the molecular formula for a compound with
  a mass of 40 amu and the empirical formula C3H4.

20
More Practice Problems

• Writing Empirical Formulas
• 1.Determine the empirical formula of a compound
  containing 2.644 g of gold and 0.476 of chlorine.

21

• 2. Determine the empirical formula of a compound
  containing 0.928 g of gallium and 0.412 of
  phosphorus.

22
Writing Molecular Formulas

1. Find the molecular formula of a compound that
   contains 42.56g of palladium and 0.80g of
   hydrogen. The molar mass of the compound is 216.
   g/mol.