Question

$$\begin{gathered} \mathbf{16}.1.22gofbenzoicacidisdissolvedin\left(i\right)100gacetone\left(K_{b}foracetone=1.7\right)and\left(ii\right)100gbenzene\left(K_{b}forbenzene=2.6\right).Theelevationinboilingpoints{T}_{b}is0.17°Cand0.13°Crespectively. \\ \left(a\right)Whatarethemolecularweightsofbenzoiczcidinboththesolutions? \\ \left(b\right)whatdoyoudeduceoutofitintermsofstructureofbenzoicacid? \end{gathered}$$

Answer 1

Dear student
(a) ∆T_b = K_b * M
In first case,
∆T_b = K_b * m = K_b* Wt.of solute/Mol. wt. of solute
0.17⁰C = 1.7 * 1.22/M *1000 * 10^-3
M = 122
Thus the benzoic acid exists as a monomer in acetone.
In second case
∆T_b = K_b * Wt. of solute/Mol. wt. of solute
0.13⁰C = 2.6 * 1.22/M’ * 100 * 10^-3
M’ = 244
b) Double the expected molecular weight of benzoic acid (244) in acetone solution indicates that benzoic acid exists as a dimer in acetone.
Regards