Long free-response question

Identifying an unknown liquid

Units 1, 2 and 3
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

A multipart question, often built around a lab setup or a data table, that mixes several topics. You calculate, explain and justify your answers across many lettered parts. On the exam: 3 questions (Questions 1–3), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

A chemist analyzes a pure, colorless liquid, compound X, that contains only carbon, hydrogen, and oxygen.

Table 1. Elemental analysis of compound X

Element	Percent by mass
C	62.0%
H	10.4%
O	27.5%

Source: Hypothetical data

Vapor measurement

A 0.512 g sample of compound X is completely vaporized in a rigid 250.0 mL flask at 100.0 °C. The pressure of the vapor in the flask is 1.08 atm. Assume the vapor behaves as an ideal gas.

Source: Hypothetical data

Table 2. Boiling points of two liquids

Liquid	Condensed formula	Molar mass (g/mol)	Normal boiling point (°C)
Acetone	(CH₃)₂CO	58.08	56
2-propanol	(CH₃)₂CHOH	60.10	82

Source: Standard values, rounded

Suggested time: 23 minutes

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Part (a)

2 points

Using the data in Table 1, calculate the number of moles of each element in a 100.0 g sample of compound X. Then determine the empirical formula of compound X.

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Part (b)

2 points

Use the vapor measurement to calculate the molar mass of compound X. Then determine the molecular formula of compound X, and justify your answer.

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Part (c)

1 point

Compound X is acetone, in which a central carbon atom is bonded to two CH₃ groups and to one oxygen atom. Describe a complete Lewis electron-dot diagram for acetone: state each bond (single, double or triple), every lone pair, and the formal charge on each atom.

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Part (d)

2 points

Identify the hybridization of the central carbon atom in acetone, and predict the approximate O–C–C bond angle around that atom. Justify your prediction.

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Part (e)

2 points

Using Table 2, explain why the boiling point of 2-propanol is higher than that of acetone. Your explanation must address the types and relative strengths of the intermolecular forces in both liquids.

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Part (f)

1 point

Acetone is miscible with water in all proportions. Explain this observation in terms of the interactions between acetone molecules and water molecules.

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