Chemical formulas were first derived by determining the masses of all the elements that combine to create a molecule, leading to the development of two important types of chemistry formulae: molecular formula and empirical formula.
The molecular formula gives the exact number of every single atom present in a molecule, whereas the empirical formula gives the simplest ratio of the number of various atoms present. It is an empirical formula if the formula is simplified. The molecular formula is a multiple of the empirical formula and is often used.
What is Molecular Formula Meaning?
The molecular formula is a formula generated from molecules that represents the total number of individual atoms in a compound's molecule. A subscript in a molecular formula indicates the number of each type of atom in a molecule of the substance.
Molecular formulas are linked to gram molecular masses, which are simple whole number multiples of the empirical formula mass.
Relationship Between Empirical and Molecular Formula
The empirical formula is the simplest formula for a compound, defined as the ratio of subscripts of the smallest whole number of elements in the formula. It's also referred to as the most basic formula.
The formula of a substance expressed with the smallest integer subscript is called an empirical formula.
The empirical formula specifies the number of atoms in the compound in a given ratio. The empirical formula of a compound is directly proportional to its % composition.
What is Meant by Molecular Formula?
Molecular Formula = n × Empirical Formula
This equation shows the molecular formula definition chemistry.
Example 1: Caffeine has the following composition: 49.48% of carbon, 5.19% of hydrogen, 16.48% of oxygen and 28.85% of nitrogen. The molecular weight is 194.19 g/mol. Find out the molecular and empirical formula.
Multiply percent composition with the molecular weight,
Carbon – 194.19 x 0.4948 = 96.0852
Hydrogen – 194.19 x 0.0519 = 10.07846
Oxygen – 194.19 x 0.1648 = 32.0025
Nitrogen – 194.19 x 0.2885 = 56.0238
Divide each value by the atomic weight,
Carbon : 96.0852 / 12.011 = 7.9997
Hydrogen : 10.07846 / 1.008 = 9.998
Oxygen : 32.0025 / 15.9994 = 2.000
Nitrogen : 56.0238 / 14.0067 = 3.9997
Round off the values to closest whole number,
Hence, the molecular formula is C8H10N4O2.
As we know 2 is the common factor of 8, 10, 4 and 2.
The empirical formula is C4H5N2O.
Example 2: The empirical formula is BH3 of boron hydride. Calculate the molecular formula given the measured mass is 27.66.
Atomic mass = B + 3(H) = 10.81 + 3(1) = 13.81u
The measured molecular mass for an atom is given as 27.66u.
Molecular formula = n × empirical formula
n = molecular formula/empirical formula = 27.6613.81 = 2
Putting the value of n = 2 in the empirical formula,
Molecular formula = 2(BH3) = B2H6.
The masses of all the elements combine to form a molecule. The empirical formula gives a ratio of the number of atoms present, but the molecular formula offers the exact number of every single atom present in a molecule. The total number of individual atoms in a compound's molecule is represented by the molecular formula.