By which method was the molecular mass of benzene determined as 78.108?

The molecular mass of benzene (C₆H₆), determined as 78.108 g/mol, was obtained using the vapour density method. This method is based on the principle that the molecular mass of a gas or vapour is twice its vapour density when compared to hydrogen under similar temperature and pressure conditions.

The relationship used is:
Molecular mass = 2 × Vapour density

In this method, the vapour density of benzene is measured experimentally by determining the mass of a known volume of benzene vapour. The measurement is made under controlled temperature and pressure to ensure accuracy. Since hydrogen is taken as the reference gas, the comparison allows for easy calculation of molecular mass.

For benzene, the experimentally found vapour density is 39.054. Using the formula, the molecular mass is calculated as:
2 × 39.054 = 78.108 g/mol.

This confirms the molecular formula of benzene as C₆H₆, where the atomic masses of carbon and hydrogen add up to 78 (6 × 12 + 6 × 1 = 78). The vapour density method was historically significant for establishing benzene’s correct molecular mass and validating its empirical formula.

Other listed methods like specific gravity, X-ray diffraction, or distillation are not suitable for molecular mass determination. The vapour density method remains one of the simplest and most direct experimental approaches for gaseous or volatile substances.

Understanding this technique helps students connect experimental gas laws with molecular weight determination in physical chemistry and provides insight into how early chemists confirmed molecular formulas accurately.