1. Empirical and Molecular Formulas

2. An empirical formula consists of the symbols for the elements combined in a compound, with the subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.

3. Calculate from Percent Composition
1. Assume a 100 g sample.
2. Convert from g to mol and round to 3 sig figs.
3. Divide molar amounts by the smallest molar amount.
4. If necessary, multiply result by a small, whole number (2,3,4) to eliminate fractions/decimals.
Example- What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

4. Calculate using sample amounts
1. Convert from g to mol and round to 3 sig figs.
2. Divide molar amounts by smallest molar amount.
3. If necessary, multiply the result by a small, whole number to eliminate fractions/decimals.
Example- Calculate the emperical formula of a compound that contains 1.67 g of cerium and g of iodine.

5. Example Problem 1
Quantitative analysis shows that a compound contains 32.83% sodium, 22.65% sulfur, and 44.99% oxygen. Find the empirical formula for this compound.

6. Example Problem 2
Analysis of a g sample of a compound known to contain only phosphorus and oxygen indicates a phosphorus content of 4.43 g. What is the empirical formula of this compound?

7. Example Problem 3
A compound is found to contain % iron and 36.48% sulfur. Find the empirical formula.

8. Molecular Formula
The molecular formula is the actual formula of a molecular compound. It is either the emperical or a multiple of it.
The relationship between a compound’s empirical formula and its molecular formula can be written as follows:
X (Empirical Formula) = Molecular Formula
X = whole-number multiple indicating the factor by which the subscripts in the empirical formula must be multiplied to obtain the molecular formula.

9. What is the molecular formula of a compound with a molecular mass of 284 g/mol and an empirical formula of P2O5?
A compound is 25% H and 75% C and has a molecular mass of 64 g/mol. What is the molecular formula?