Ch.11 - Liquids and Intermolecular Forces

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.11 - Liquids and Intermolecular Forces

Problem 75e

Chapter 11, Problem 75e

Suppose you have two colorless molecular liquids, one boiling at - 84 °C, the other at 34 °C, and both at atmospheric 6 pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (e) At - 84 °C both liquids have vapor pressures of 760 mm Hg.

Verified step by step guidance

Identify the boiling point of each liquid: Liquid A boils at -84 °C and Liquid B boils at 34 °C.

Understand that the boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure, typically 760 mm Hg at atmospheric pressure.

Since Liquid A boils at -84 °C, its vapor pressure is 760 mm Hg at this temperature.

Since Liquid B boils at 34 °C, its vapor pressure is 760 mm Hg at this temperature.

Therefore, at -84 °C, only Liquid A has a vapor pressure of 760 mm Hg, not both liquids.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Boiling Point and Vapor Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure. At this point, the liquid transitions to a gas. For the liquid boiling at -84 °C, it will have a vapor pressure of 760 mm Hg at that temperature, but the other liquid, boiling at 34 °C, will not necessarily have the same vapor pressure at -84 °C.

Vapor Pressure and Temperature Relationship

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. As temperature increases, the vapor pressure of a liquid also increases. Therefore, at -84 °C, the vapor pressures of both liquids will be lower than 760 mm Hg, making the statement about both having the same vapor pressure at that temperature incorrect.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the atmosphere at a given location, typically measured at sea level as 760 mm Hg. This pressure is crucial for understanding boiling points and vapor pressures, as liquids will boil when their vapor pressure matches this atmospheric pressure. The statement in question fails to consider that vapor pressures vary with temperature and are not constant across different liquids.

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Textbook Question

Suppose you have two colorless molecular liquids, one boiling at - 84 °C, the other at 34 °C, and both at atmospheric 6 pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (d) The two liquids have identical vapor pressures at their normal boiling points.

Textbook Question

The table below lists the density of O₂ at various temperatures and at 1 atm. The normal melting point of O₂ is 54 K.

(b) Over what temperature range is O₂ a liquid?

Textbook Question

Two isomers of the planar compound 1,2-dichloroethylene are shown here.

(a) Which of the two isomers will have the stronger dipole– dipole forces?

Textbook Question

The table below shows the normal boiling points of benzene and benzene derivatives.

(a) How many of these compounds exhibit dispersion interactions?

Textbook Question

The table below shows the normal boiling points of benzene and benzene derivatives. (e) Why is the boiling point of phenol the highest of all?