# Writing Empirical and Molecular Formulas - PowerPoint PPT Presentation

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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

### 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:

• 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:

Some formulas, such as the one for carbon

dioxide, CO2, are already empirical formulas

without being reduced.

### 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.

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:

• 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.

• 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:

• A substance is 36.1% by weight calcium

and 63.9% chlorine. What is the empirical

formula of this compound?

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

45.3% O. What is the empirical formula for

this compound?

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?

• 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:

• 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

### 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 Problems:

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

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

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

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

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

### 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.

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

### 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.