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AP Chemistry 3.3 Solids, Liquids, and Gases- Exam Style questions - FRQs- New Syllabus

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Question

A scientist is investigating the properties of a mixture of \( \mathrm{CH_3OH} \) and \( \mathrm{H_2CO} \). The scientist generates the following diagram to represent the mixture.
A. Identify the hybridization of the valence orbitals of the \( \mathrm{C} \) atom in the \( \mathrm{H_2CO} \) molecule.
B. In the diagram provided, draw a SINGLE dashed line \((—-)\) to represent a strong hydrogen-bonding attraction between one \( \mathrm{CH_3OH} \) molecule and one \( \mathrm{H_2CO} \) molecule in the mixture.
C. The scientist plans to cool a gaseous mixture of \( \mathrm{CH_3OH} \) and \( \mathrm{H_2CO} \) to form a liquid mixture and finds data on the two compounds. The data are summarized in the table.
SubstanceMelting Point \((K)\)Boiling Point \((K)\)Enthalpy of Vaporization \((\mathrm{kJ/mol})\)
\( \mathrm{CH_3OH} \)\(176\)\(338\)\(37.6\)
\( \mathrm{H_2CO} \)\(181\)\(254\)\(24.2\)
i. Propose a temperature to which the mixture should be cooled such that \( \mathrm{CH_3OH} \) and \( \mathrm{H_2CO} \) will both be liquids.
ii. The scientist analyzes the mixture after it is cooled and determines that \(8.59\ \mathrm{g}\) of \( \mathrm{CH_3OH(l)} \) is present. Calculate the amount of thermal energy, in \( \mathrm{kJ} \), that was removed to condense the \(8.59\ \mathrm{g}\) of \( \mathrm{CH_3OH} \) \((\mathrm{molar\ mass}=32.04\ \mathrm{g/mol})\) at its boiling point.

Most-appropriate topic codes (AP Chemistry):

• Topic \(2.7\) — VSEPR and Hybridization — Part A
• Topic \(3.1\) — Intermolecular and Interparticle Forces — Part B
• Topic \(3.3\) — Solids, Liquids, and Gases — Part C(i)
• Topic \(6.5\) — Energy of Phase Changes — Part C(ii)
▶️ Answer/Explanation

A
For the correct answer:
\( \mathrm{sp^2} \)
The \( \mathrm{C} \) atom in \( \mathrm{H_2CO} \) has three regions of electron density around it: two single bonds and one double bond.
A molecule with three electron domains around the central atom has \( \mathrm{sp^2} \) hybridization.

B
For a correct diagram:
The diagram should show a dashed line between the \( \mathrm{O} \) atom in one \( \mathrm{H_2CO} \) molecule and the \( \mathrm{H} \) atom in the \( \mathrm{-OH} \) group of one \( \mathrm{CH_3OH} \) molecule. See example response below.
This is because hydrogen bonding occurs when an \( \mathrm{H} \) attached to \( \mathrm{O} \) is attracted to a lone pair on another electronegative atom such as \( \mathrm{O} \).

C(i)
For a correct proposal:
The proposed temperature should be in the range \(181\ \mathrm{K} – 254\ \mathrm{K}\).
To be liquids, the temperature must be above each substance’s melting point and below each substance’s boiling point.
Any temperature in this overlap range is acceptable, for example \(200\ \mathrm{K}\).

C(ii)
For the correct calculated value:

\[ 8.59\ \mathrm{g\ CH_3OH} \times \frac{1\ \mathrm{mol\ CH_3OH}}{32.04\ \mathrm{g\ CH_3OH}} \times \frac{-37.6\ \mathrm{kJ}}{1\ \mathrm{mol\ CH_3OH}} = -10.1\ \mathrm{kJ} \]

First convert grams of \( \mathrm{CH_3OH} \) to moles, then multiply by the enthalpy of vaporization.
The negative sign shows energy is released by the substance during condensation.
Therefore, \(10.1\ \mathrm{kJ}\) are removed.

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