Benzoic acid (molar mass 122 g mol^-1) associates into dimers in benzene. When 6.1 g of benzoic acid is dissolved in 100 g of benzene (taking Kf=5.12…

Molality is $m=\dfrac{6.1/122}{0.100\ \mathrm{kg}}=0.500\ \mathrm{mol\,kg^{-1}}$. If the observed depression is $\Delta T_f=i\,K_fm$, then $i=\dfrac{1.38}{5.12\times0.500}=0.5391$. For association $2\mathrm{A}\rightleftharpoons \mathrm{A_2}$ with a fraction $\alpha$ of the molecules dimerised, $1$ mol of monomer gives $(1-\alpha)+\alpha/2=1-\alpha/2$ mol of particles, so $i=1-\alpha/2$. Therefore $\alpha=2(1-i)=2(1-0.5391)=0.9219$. The fraction of molecules present as dimers is thus $92.19\%$. The value of $i$ is below one because the number of solute particles falls on association, which is the opposite of the dissociation case where $i>1$. Had the acid not associated, $i$ would be $1$ and the depression would be $2.56\ \mathrm{K}$; the observed value, a little over half of that, shows that the molecules are almost completely present as hydrogen-bonded dimers, which is the reason the apparent molar mass in benzene is nearly double $122$.