Q78P - Chemistry: Molecular Nature Of Matter And Change

Question

What is the difference between an empirical formula and a molecular formula? Can they ever be the same?

Step-by-Step Solution

Answer

The empirical formula is the simplest ratio of the atoms, whereas the molecular formula determines the actual number of atoms present in the compound. The compound whose molecular formula cannot be simplified further depicts the same empirical and molecular formula.

Step 1: Determine the empirical formula of the compound

In the compound, the empirical formula depicts the relative number of atoms of each element present in that particular compound. For example, in ethylene, two carbon and four hydrogen atoms are present. Therefore, the empirical formula of ethylene is ${\mathbf{CH}}_{\mathbf{2}}$.

Step 2: Define the molecular formula of the compound

The molecular formula depicts the actual number of atoms present in that particular compound. For example, in ethylene, two carbon and four hydrogen atoms are present. Therefore, the empirical formula of ethylene is ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}$ .

Step 3: Difference between an empirical and molecular formula of the compound

The empirical formula is the simplest ratio of the atoms present in that compound, whereas the molecular formula determines the actual number of atoms present in the compound. For example, in glucose, six-carbon, twelve hydrogen, and six oxygen atoms are present.

 

The ratio of the atoms present in the glucose is C: H: O is 6: 12: 6. Therefore, the simplest ratio of glucose is C: H: O is 1: 2: 1. Hence, the empirical formula for the glucose is ${\mathbf{CH}}_{\mathbf{2}}\mathbf{O}$

 

The molecular formula depicts the actual number of elements present in that particular compound. For example, in glucose six-carbon, twelve hydrogen, and six oxygen atoms are present. Therefore, the molecular formula of glucose is ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}{\mathbf{O}}_{\mathbf{6}}$

Step 4: Identification of compounds having the same molecular and empirical formula

There are different compounds, whose empirical formula is the same as their molecular formula. In these compounds, the molecular formula cannot be simplified further. For example, in water two hydrogen and oxygen atoms are present. So, the molecular formula of water is ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$ since the molecular formula of water is already present in its simplest form and cannot be further simplified, its empirical formula is also the same. Hence, the molecular and empirical formula of water is  the same, that is, ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$

 

Likewise, carbon dioxide has the same molecular and empirical formula, that is, ${\mathbf{CO}}_{\mathbf{2}}$.