Why `AlCl_3` exist as dimers?

Boron chloride do not form dimers where as Aluminium chloride exist as dimers. Do you agree with this statement? Substantiate your answer.

Why does formic acid exist as dimer? What is its one consequence?

A compound exist in gaseous state both as monomer and dimer. The molecular mass of monomer is 48. In an experiment 96 g of the compound was taken in a vessel of volume 33.6 L at 273 K. What is the pressure developed if the compound exist as a dimer to an extend of 50% by mass?