Writing empirical and molecular formulas
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Writing Empirical and Molecular Formulas. 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. Example:.

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Writing Empirical and Molecular Formulas

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Writing empirical and molecular formulas

Writing Empirical and Molecular Formulas


What is an empirical formula

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.


Example

Example:

  • The empirical formula for a glucose molecule (C6H12O6) is CH2O. All the subscripts are divisible by six.

    C6H12O6

    6 6 6

    C H2 O


Exceptions

Exceptions:

Some formulas, such as the one for carbon

dioxide, CO2, are already empirical formulas

without being reduced.


Calculating empirical formula

Calculating empirical formula:

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


Determine the empirical formulas for each of the following molecular formulas

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….___________


Finding empirical formulas using molecular mass

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.


Example find the empirical formula for a sample of 25 9 n and 74 1 o

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


Practice problems

PRACTICE PROBLEMS:

  • A substance is 36.1% by weight calcium

    and 63.9% chlorine. What is the empirical

    formula of this compound?


Writing empirical and molecular formulas

2. A compound is 43.4%Na, 11.3%C, and

45.3% O. What is the empirical formula for

this compound?


Writing empirical and molecular formulas

3. A compound is 2.46%H, 39.1%S, and

58.5%O. What is the empirical formula for

this compound?


What is a molecular formula

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.


To find the molecular formula from the empirical formula

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


Example1

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


Practice problems1

Practice Problems:

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


Writing empirical and molecular formulas

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


Writing empirical and molecular formulas

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


Writing empirical and molecular formulas

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


Writing empirical and molecular formulas

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


More practice problems

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.


Writing empirical and molecular formulas

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


Writing molecular formulas

Writing Molecular Formulas:

  • 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.


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