Question

Naphthalene contains 93.71% carbon and 6.29% hydrogen. If its molar mass is 128 g mole^-1 , then its molecular formula will be ­
1. C₈H₈
2. C₁₀H₈
3. C₁₀H₁₀
4. C₆H₆

Answer 1

Correct Answer - Option 2 : C₁₀H₈

The correct answer is C₁₀H₈.

• Naphthalene contains 93.71% carbon and 6.29% hydrogen.
  • If its molar mass is 128 g mole^-1 , then its molecular formula will be ­C10H8.

Assume the total mass is 100 grams

Mass of each element = percent given

⇒Mass of carbon = 93.71

⇒Mass of hydrogen = 6.29

Dividing each proportion by the atomic weight of the component

⇒Number of mole (carbon) = 93.71/12​ = 7.8 relative moles of carbon

⇒Number of mole (hydrogen) = 6.29/1 = 6.29 relative moles of hydrogen

Hence most simple ratio,

⇒for carbon 7.8/6.29​ = 1.25 

⇒For hydrogen 6.29/6.29 = 1​

⇒Since it is not a whole number we multiply the ratio by 4 to get a whole number ratio.

The lowest whole number ratio is

⇒for carbon = 5

⇒for hydrogen = 4

⇒Empirical formula = C₅​H₄

​⇒Ratio = 128g/64g ​= 2

⇒Molecular formula = 2×C₅​H₄ ​= C10H8

• A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule.
• The empirical formula represents the simplest whole-integer ratio of atoms in a compound. The molecular formula for a compound can be the same as or a multiple of the compound’s empirical formula.
• Molecular formulas are compact and easy to communicate; however, they lack the information about bonding and atomic arrangement that is provided in a structural formula.