ChemistryEmpirical and Molecular Formulas

Objectives Students will be able to: Determine the empirical formula of a

compound Determine the molecular formula of a

compound

Two types of formulasEmpirical Formula Gives the formula of a

compound in the simplest whole number ratio of atoms/moles in a compound

Molecular Formula “True Formula”—gives

the actual ratio of atoms/moles in a compound

B. Empirical Formula

C2H6

CH3

reduce subscripts

Smallest whole number ratio of atoms in a compound

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. C2H4O2….___________

• C2H9

• HO• HgCl• CH2

• NaCO2

• H2O• CH2

• C2H3

• C7H12

• CH2O

Finding Empirical Formula1. Use the % of each element as its mass in the compound (ex 36.8% N, 63.2%O)

2. Convert mass of each element to moles.

3. Divide moles by the smallest # of moles to find subscripts.

4. If #’s are not whole #’s, multiply subscripts by 2, 3, or 4 to get whole #’s.

B. Empirical Formula Find the empirical formula for a

sample of 25.9% N and 74.1% O.

25.9 g 1 mol14.0 g

= 1.85 mol N

74.1 g 1 mol16.0 g

= 4.63 mol O

1.85 mol

1.85 mol

= 1 N

= 2.5 O

B. Empirical Formula

N1O2.5Need to make the subscripts whole

numbers multiply by 2

N2O5

Steps for finding Molecular Formula1. Determine the empirical formula(either given or you find it)

2. Find the molar mass of the empirical formula3. Divide the Molecular Formula mass (given in problem)

by the EF mass to create a ratio (n).

4. Multiply the EF formula by the whole # ratio (n). n= MFmass molecular formula = (EF)n

EFmass

Molecular formula: can be the same as the empirical formula or a

Whole # multiple of the empirical.

It is the true number of atoms of each element in a compound.

Finding the Molecular Formula

The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?Step 1 (given in problem) Determine Empirical Formula

Step 2. Find the Molar mass of CStep 2. Find the Molar mass of C33HH55OO22

3(12.0 g) + 5(1.0) + 2(16.0) = 73.0 g3(12.0 g) + 5(1.0) + 2(16.0) = 73.0 g

Finding the Molecular FormulaThe empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?

3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g

146 273

Finding the Molecular FormulaThe empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?

3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g

146 273

Step 4. Multiply the empirical formula by this number to get the molecular formula.

(C3H5O2) x 2 = C6H10O4