Question

The freezing point of benzene decreases by $0.{45}^{°}\mathrm{C}$when $0.2\mathrm{g}$acetic acid is added to$20\mathrm{g}$ of benzene. If acetic acid associates to form a dimer in benzene, the percentage association of acetic acid in benzene will be : $(\mathrm{Kf}\mathrm{for}\mathrm{benzene}=5.12\mathrm{K}\mathrm{kg}{\mathrm{mol}}^{-1})$

• A. $74.6\%$
• B. $94.6\%$
• C. $64.6\%$
• D. $80.4\%$

Answer 1

The correct option is B

$94.6\%$

The correct option is (b)

The Explanation for the correct option:

(b)

• Explanation: The benzene molecule is made up of six carbon atoms joined together in a planar ring, each with one hydrogen atom attached to it. Benzene is classified as a hydrocarbon because it only contains carbon and hydrogen atoms.
• Benzene's normal melting and boiling points are $5.{55}^{°}\mathrm{C}\mathrm{to}78.25°\mathrm{C}$respectively (at the normal melting and boiling point the pressure is one atm).

$$\begin{gathered} 2{\mathrm{CH}}_3\mathrm{COOH}\rightleftharpoons {\left[{\mathrm{CH}}_3\mathrm{COOH}\right]}_2 \\ (1–\mathrm{\alpha })\mathrm{\alpha }/2 \\ \mathrm{i}=(1–\mathrm{\alpha })/2 \\ 0.527=1–(\mathrm{\alpha }/2) \\ \mathrm{\alpha }/2=0.473 \\ \mathrm{\alpha }=0.946 \\ \mathrm{\alpha }=94.6\% \end{gathered}$$

The Explanation for the incorrect option:

(a)

• Explanation: The heterocyclic molecule is made up of six carbon atoms arranged in a planar ring, with each atom attached to one hydrogen atom.
• Despite the fact that it only contains alkenes, benzene is classified as a hydrocarbon.

(c)

• Explanation: The benzyl molecule is composed of six carbon atoms joined in a planar ring, each with one hydrogen atom attached.
• When benzene contains only carbon and hydrogen atoms, it is classified as a hydrocarbon.

(d)

• Explanation: The benzene molecule is made up of six carbon atoms linked in a planar ring, each with two hydrogen atoms attached.
• Because it is entirely composed of hydrocarbon chains, formaldehyde is classified as petroleum.

The correct response is (b): The benzene molecule is composed of six carbon atoms linked in a planar ring, with two hydrogen atoms attached to each.