Just above its boiling point, gaseous ethanoicacid has an apparent relative molecular mass of 120 instead of the expected 60. This CH, C C CH, is because the gas particles consist of two CH CooH joined together by hydrogen bonds (Figure 3.33). This joining together of molecules in pairs is called dimerisation. Figure 3.33 Dimerisation of ethanoic acid Gaseous hydrogen fluoride has a larger apparent relative molecular mass than expected because the molecules are joined in twos or threes, i e (HF)2 or (HF) in the gaseous state by hydrogen bonding.