Empirical and molecular formula

Molecular Formula

The molecular formula is an expression of the number and type of atoms that are present in a single molecule of a substance.

Empirical Formula

The empirical formula is also known as the simplest formula. The empirical formula is the ratio of elements present in the compound.

Examples of Molecular and Empirical Formulas

The molecular formula of glucose is: C6H12O6. One molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen. If you can divide all of the numbers in molecular formula by some value to simplify them further, then the empirical or simple formula will be different from the molecular formula. The empirical formula for glucose is CH2O. Glucose has 2 moles of hydrogen for every mole of carbon and oxygen.

Key Concepts

• Empirical Formula of a compound shows the lowest whole number ratio of elements present in a compound.

• Molecular Formula of a compound shows how many atoms of each element are present in a molecule of the compound.

• The empirical formula mass of a compound refers to the sum of the atomic masses of the elements present in the empirical formula.

• The Molecular Mass (formula mass, formula weight or molecular weight) of a compound is a multiple of the empirical formula mass.

Mr = n × empirical formula mass

where Mr = molecular mass

and n = a whole number (1 ,2 3, etc)

Alternatively, you can relate the molar mass of the molecular formula and the empirical formula in the same way:

molar mass of molecular formula = n × molar mass of empirical formula

• Empirical Formula can be calculated from the percentage (or percent) composition of a compound.

Steps for Determining an Empirical Formula

1. Start with the number of grams of each element, given in the problem. If percentages are given, assume that the total mass is 100 grams so that :

the mass of each element = the percent given.

2. Convert the mass of each element to moles using the molar mass from the periodic table.

3. Divide each mole value by the smallest number of moles calculated.

4. Round to the nearest whole number. This is the mole ratio of the elements and is represented by subscripts in the empirical formula.