6.7 MOLECULAR AND STRUCTURAL FORMULA

The structural formula of a compound shows the arrangement of atoms present in it, while the molecular formula shows the number of atoms of each element. For example, the structural formula of n-Butane is CH₃-CH₂-CH₂-CH₃ and its molecular formula is C₄H₁₀.

To write the molecular formula of a compound from its structural formula, follow these steps:
1. Identify different types of elements present in the structural formula.
2. Write symbols of these elements side by side in a line.
3. Count the number of atoms of each element from the structural formula.
4. Show this number of atoms as subscripts of the symbol of the corresponding element.

Example 6.13: Writing the Molecular Formula

Given the structural formula: CH₃-CH₂-CH₂-OH
The molecular formula is: C₃H₈O

CONCEPT ASSESSMENT EXERCISE 6.8

Write the molecular formulae of the following compounds:
1. CH₃-CH₂-OH
2. CH₃-CH₂-NH₂
3. CH₃-CO-CH₃

KEY POINTS

• Chemistry is the science of materials of the universe.
• The branch of Chemistry that deals with laws and theories to understand the structure and changes of matter is called Physical Chemistry.
• An element is a substance all the atoms of which have the same atomic number.
• A compound consists of two or more elements held together in fixed proportions by chemical bonds.
• Chemical formula of a compound that gives the simplest whole-number ratio between atoms is called empirical formula.
• Molecular formula of a compound gives the exact number of atoms present in a molecule.
• Molecular mass is the sum of atomic masses of all the atoms present in the molecule.
• The number of representative particles in one mole of the substance is known as Avogadro's number.
• The amount of matter that contains as many atoms, ions, or molecules as the number of atoms in exactly 12g of C-12 is called mole. Mole can also be defined as atomic mass, molecular mass, or formula mass expressed in grams.
• Atomic mass of an element expressed in grams is called gram atomic mass.
• Molecular mass of an element or a compound expressed in grams is its gram molecular mass.
• Gram formula mass is the formula mass of a substance in grams.

References for Additional Information

• Zumdahl, Introductory Chemistry.
• Raymond Chang, Essential Chemistry.