Why the relative molecular mass of aluminium chloride in the vapour phase is twice the expected value?

The relative molecular mass of aluminium chloride (AlCl₃) in the vapor phase is typically found to be about twice the expected value because it exists as a dimer (Al₂Cl₆) rather than as individual AlCl₃ molecules. In the gaseous state, two AlCl₃ units can associate to form this dimer due to intermolecular forces. This dimerization leads to the observed molecular mass being twice that of the monomeric AlCl₃, thus explaining the discrepancy in the expected value.