Question

How to calculate the size of the molecule;

1)Ethane (CH3CH3)

2)fluoromethane (CH3F)

explaination with the law/formula used

Answer 1

Nowadays there are advanced techniques(spectroscopic methods, X-ray crystallography etc) to determine the size of a molecule. To get a rough idea about the size of a molecule we can simply use the formula, V_molecule = V_molar/ N_A

You can get molar volume (V_molar) from the knowledge of molar mass(M) and density(d) : V_molar = M/d

1. Ethane: M= 30.07 g/mol ; d = 1.28 g/cm³

V_molar = M/d = 23.98 cm³/mol

V_molecule = V_molar/ N_A = (23.98 cm³/mol) / ( 6.023*10²³ molecules/mol) = 3.98 * 10^-23 cm³/molecule

2. fluoromethane : M= 34.03 g/mol ; d = 1.44 g/cm³

V_molar = M/d = 23.63 cm³/mol

V_molecule = V_molar/ N_A = (23.63 cm³/mol) / ( 6.023*10²³ molecules/mol) = 3.92 * 10^-23 cm³/molecule