The molecular formula mass is a whole number multiple of the empirical formula mass.
Here's a more detailed explanation:
The relationship between the molecular formula and the empirical formula is fundamental in chemistry. Understanding this relationship allows us to determine the actual number of atoms of each element present in a molecule, as opposed to just the simplest whole-number ratio.
Empirical Formula and Empirical Formula Mass
The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound. For example, the empirical formula of glucose is CH2O. The empirical formula mass is the sum of the atomic masses of all the atoms in the empirical formula. For CH2O, the empirical formula mass is approximately 12 (C) + 2(1) (H) + 16 (O) = 30 amu (atomic mass units).
Molecular Formula and Molecular Formula Mass
The molecular formula represents the actual number of atoms of each element in a molecule of the compound. The molecular formula of glucose is C6H12O6. The molecular formula mass (also known as molecular weight or molar mass) is the sum of the atomic masses of all the atoms in the molecular formula. For C6H12O6, the molecular formula mass is approximately 6(12) (C) + 12(1) (H) + 6(16) (O) = 180 amu.
The Relationship
The molecular formula is always a whole-number multiple of the empirical formula. Mathematically:
Molecular Formula = n × Empirical Formula
Where 'n' is a simple integer (1, 2, 3, etc.).
Consequently, the molecular formula mass is 'n' times the empirical formula mass:
Molecular Formula Mass = n × Empirical Formula Mass
Therefore:
n = Molecular Formula Mass / Empirical Formula Mass
This 'n' value allows you to determine the molecular formula if you know the empirical formula and the molecular formula mass.
Example
Let's revisit glucose (C6H12O6). We determined that the empirical formula is CH2O and the molecular formula mass is 180 amu, and the empirical formula mass is 30 amu.
n = 180 amu / 30 amu = 6
Therefore, the molecular formula = 6 × (CH2O) = C6H12O6.
Summary
The molecular formula mass provides information about the actual number of atoms in a molecule, while the empirical formula mass relates to the simplest ratio. The molecular formula mass is always a whole-number multiple ('n') of the empirical formula mass. Determining 'n' allows you to calculate the molecular formula from the empirical formula.
