Name: Period: Date:

Empirical and Molecular Formulas

What are molecular formulas?

Molecular formulas tell the number of atoms of each element in a compound.

Example: Glucose – C6H12O6 (one molecule of glucose has 6 carbons 12 hydrogens and 6 oxygens).

What are empirical formulas?

Empirical formulas are the ratio of elements in a compound. It’s the chemical formula.

Example: Benzene – C6H6 (molecular formula)  CH (empirical formula)

Water – H2O (molecular formula)  H2O (empirical formula)

The empirical formula be the same as the molecular formula.

Practice

Dinitrogen tetroxide Tetraphosphorus decoxide

Molecular Formula: Molecular Formula:

Empirical Formula: Empirical Formula:

How do you calculate empirical formulas?

Remember this rhyme…

Percent to Mass

Mass to Mole

Divide by Small

Multiply ‘til Whole

1. Assume you have 100g of the sample. the percent of each element by 100g to determine the amount in of each element.

2. Convert grams to for each element.

3. all answers by the smallest number of moles.

4. If needed, multiply all numbers by the same number to get numbers. Your final answer

5. The whole numbers are the of the empirical formula.

Practice

A compound is found to be 34.53% zinc, 14.79% nitrogen, and 50.68% oxygen. Calculate the empirical formula for this compound.

How do you find the molecular formula from the empirical formula?

1. Calculate the for the empirical formula.

2. Find the molar mass given in the .

3. Find the “magic number” n

4. each subscript in the empirical formula by n (“magic number”).

5. Write the formula with the OR in the order in which it was given if there is no metal.

Practice

The empirical formula for a compound is CHO. The molar mass is 116.1 g/mol. Determine the molecular formula.

Empirical Formula Practice Problems

1. A compound is found to contain 63.52% iron and 36.48% sulfur. Find its empirical formula.

2. The percent composition of a compound was found to be 63.5% silver, 8.2% nitrogen, and 28.3% oxygen. Determine its empirical formula.

3. A compound containing 5.92% hydrogen and 94.08% oxygen has a molar mass of 34.01 g/mol. Determine the empirical and molecular formula of this compound.