Answer
See the answer below.
Work Step by Step
Benzene solution:
$\Delta T_f=K_fm\rightarrow m=(3.37-5.50)^{\circ}C\div -5.12^{\circ}C/m=0.416\ m$
Water solution:
$\Delta T_f=K_fm\rightarrow m=(-1.49-0)^{\circ}C\div -1.86^{\circ}C/m=0.801\ m$
Since the ratio of the two is approximately two and acetic acid displays hydrogen bonding, in the benzene solution its prevailing form is probably dimeric.
