Chemical Calculations Lesson # 4 – Empirical and Molecular Formula

Definitions If we have the percent composition of an unknown compound, we can determine the ratio of elements in this compound through experiment (determined empirically, versus theoretically). The empirical formula of a compound is the simplest whole-number ratio of the atoms (or ions) in the compound. The molecular formula gives the actual number of atoms of each element in one molecule of the compound.

Glucose Molecular Formula: C6H12O6 Empirical Formula:

Example 1 What is the empirical formula of a compound, which is found by analysis to contain 92.3% C and 7.7% H?

Example 2 What is the empirical formula of a compound, which on analysis shows 2.2% H, 26.7% C, and 71.1% O?

Example 4 What is the empirical formula of a compound found to contain 69.9% Fe and 30.1% O?

Molecular Formula The molecular formula of a compound tells you the exact number of atoms in one molecule of the compound. The molecular formula may be equal to the empirical formula (the formula for water is H2O, and it cannot be reduced), or it may be a multiple of the formula (as we saw with glucose). You cannot determine the molecular formula without first knowing the empirical formula.

Example 5 CH2O is the empirical formula of a certain compound whose molecular mass is 180 g/mol. What is the molecular formula of the compound?

Example 6 A compound is made up of 21.9% Na, 45.7% C, 1.9% H, and 30.5% O. What is the molecular formula if the molecular mass is 210 g/mol?